Please post output of qdbus org.kde.KWin /KWin supportInformation

Also, please try to get a backtrace with debug symbols.

if i understand things correctly (and it's likely i don't, i'm a noob), I would have to complie the package with debug flags enabled and reproduce the bug. i will try to do so, but i have been unable to reproduce the bug so far.

qdbus org.kde.KWin /KWin supportInformation:

KWin Support Information:
The following information should be used when requesting support on e.g. https://forum.kde.org.
It provides information about the currently running instance, which options are used,
what OpenGL driver and which effects are running.
Please post the information provided underneath this introductory text to a paste bin service
like http://paste.kde.org instead of pasting into support threads.

==========================

Version
=======
KWin version: 5.15.4
Qt Version: 5.12.2
Qt compile version: 5.12.2
XCB compile version: 1.13.1

Operation Mode: X11 only

Build Options
=============
KWIN_BUILD_DECORATIONS: yes
KWIN_BUILD_TABBOX: yes
KWIN_BUILD_ACTIVITIES: yes
HAVE_DRM: yes
HAVE_GBM: yes
HAVE_X11_XCB: yes
HAVE_EPOXY_GLX: yes
HAVE_WAYLAND_EGL: yes

X11
===
Vendor: The X.Org Foundation
Vendor Release: 12004000
Protocol Version/Revision: 11/0
SHAPE: yes; Version: 0x11
RANDR: yes; Version: 0x14
DAMAGE: yes; Version: 0x11
Composite: yes; Version: 0x4
RENDER: yes; Version: 0xb
XFIXES: yes; Version: 0x50
SYNC: yes; Version: 0x31
GLX: yes; Version: 0x0

Decoration
==========
Plugin: org.kde.breeze
Theme: 
Blur: 0
onAllDesktopsAvailable: true
alphaChannelSupported: true
closeOnDoubleClickOnMenu: false
decorationButtonsLeft: 0, 9, 1
decorationButtonsRight: 6, 3, 4, 5
borderSize: 3
gridUnit: 10
font: Noto Sans,10,-1,0,50,0,0,0,0,0,Regular
smallSpacing: 2
largeSpacing: 10

Platform
==========
Name: KWin::X11StandalonePlatform

Options
=======
focusPolicy: 0
nextFocusPrefersMouse: false
clickRaise: true
autoRaise: false
autoRaiseInterval: 0
delayFocusInterval: 0
shadeHover: false
shadeHoverInterval: 250
separateScreenFocus: false
placement: 4
focusPolicyIsReasonable: true
borderSnapZone: 10
windowSnapZone: 10
centerSnapZone: 50
snapOnlyWhenOverlapping: false
rollOverDesktops: true
focusStealingPreventionLevel: 1
legacyFullscreenSupport: false
operationTitlebarDblClick: 5000
operationMaxButtonLeftClick: 5000
operationMaxButtonMiddleClick: 5015
operationMaxButtonRightClick: 5014
commandActiveTitlebar1: 0
commandActiveTitlebar2: 30
commandActiveTitlebar3: 2
commandInactiveTitlebar1: 4
commandInactiveTitlebar2: 30
commandInactiveTitlebar3: 2
commandWindow1: 7
commandWindow2: 8
commandWindow3: 8
commandWindowWheel: 31
commandAll1: 10
commandAll2: 3
commandAll3: 14
keyCmdAllModKey: 16777251
showGeometryTip: true
condensedTitle: false
electricBorderMaximize: true
electricBorderTiling: true
electricBorderCornerRatio: 0.25
borderlessMaximizedWindows: false
killPingTimeout: 5000
hideUtilityWindowsForInactive: true
inactiveTabsSkipTaskbar: false
autogroupSimilarWindows: false
autogroupInForeground: true
compositingMode: 1
useCompositing: true
compositingInitialized: true
hiddenPreviews: 1
glSmoothScale: 2
xrenderSmoothScale: false
maxFpsInterval: 16666666
refreshRate: 0
vBlankTime: 6000000
glStrictBinding: true
glStrictBindingFollowsDriver: true
glCoreProfile: true
glPreferBufferSwap: 101
glPlatformInterface: 1
windowsBlockCompositing: true

Screen Edges
============
desktopSwitching: false
desktopSwitchingMovingClients: false
cursorPushBackDistance: 1x1
timeThreshold: 150
reActivateThreshold: 350
actionTopLeft: 0
actionTop: 0
actionTopRight: 0
actionRight: 0
actionBottomRight: 0
actionBottom: 0
actionBottomLeft: 0
actionLeft: 0

Screens
=======
Multi-Head: no
Active screen follows mouse:  no
Number of Screens: 1

Screen 0:
---------
Name: eDP1
Geometry: 0,0,1920x1080
Scale: 1
Refresh Rate: 60.0204


Compositing
===========
Compositing is active
Compositing Type: OpenGL
OpenGL vendor string: Intel Open Source Technology Center
OpenGL renderer string: Mesa DRI Intel(R) HD Graphics 5500 (Broadwell GT2) 
OpenGL version string: 4.5 (Core Profile) Mesa 19.0.2
OpenGL platform interface: GLX
OpenGL shading language version string: 4.50
Driver: Intel
GPU class: Unknown
OpenGL version: 4.5
GLSL version: 4.50
Mesa version: 19.0.2
X server version: 1.20.4
Linux kernel version: 5.0.7
Direct rendering: Requires strict binding: yes
GLSL shaders:  yes
Texture NPOT support:  yes
Virtual Machine:  no
OpenGL 2 Shaders are used
Painting blocks for vertical retrace:  no

Loaded Effects:
---------------
zoom
snaphelper
kwin4_effect_fade
mousemark
kwin4_effect_fadingpopups
kwin4_effect_dialogparent
kwin4_effect_logout
kwin4_effect_maximize
kwin4_effect_translucency
kwin4_effect_morphingpopups
kwin4_effect_frozenapp
kwin4_effect_login
slidingpopups
screenshot
flipswitch
diminactive
desktopgrid
cubeslide
coverswitch
colorpicker
presentwindows
highlightwindow
blur
contrast
windowgeometry
startupfeedback
screenedge
kscreen

Currently Active Effects:
-------------------------
diminactive
blur
contrast

Effect Settings:
----------------
zoom:
zoomFactor: 1.2
mousePointer: 0
mouseTracking: 0
enableFocusTracking: false
followFocus: true
focusDelay: 350
moveFactor: 20
targetZoom: 1

snaphelper:

kwin4_effect_fade:
isActiveFullScreenEffect: false

mousemark:
width: 3
color: #ff0000

kwin4_effect_fadingpopups:
isActiveFullScreenEffect: false

kwin4_effect_dialogparent:
isActiveFullScreenEffect: false

kwin4_effect_logout:
isActiveFullScreenEffect: false

kwin4_effect_maximize:
isActiveFullScreenEffect: false

kwin4_effect_translucency:
isActiveFullScreenEffect: false

kwin4_effect_morphingpopups:
isActiveFullScreenEffect: false

kwin4_effect_frozenapp:
isActiveFullScreenEffect: false

kwin4_effect_login:
isActiveFullScreenEffect: false

slidingpopups:
slideInDuration: 1
slideOutDuration: 1

screenshot:

flipswitch:
tabBox: false
tabBoxAlternative: false
duration: 1
angle: 30
xPosition: 0.33000001311302185
yPosition: 1
windowTitle: true

diminactive:
dimStrength: 25
dimPanels: false
dimDesktop: false
dimKeepAbove: false
dimByGroup: true
dimFullScreen: true

desktopgrid:
zoomDuration: 1
border: 10
desktopNameAlignment: 0
layoutMode: 0
customLayoutRows: 2
usePresentWindows: true

cubeslide:
rotationDuration: 1
dontSlidePanels: true
dontSlideStickyWindows: false
usePagerLayout: true
useWindowMoving: false

coverswitch:
animationDuration: 1
animateSwitch: true
animateStart: true
animateStop: true
reflection: true
windowTitle: true
zPosition: 900
primaryTabBox: false
secondaryTabBox: false

colorpicker:

presentwindows:
layoutMode: 0
showCaptions: true
showIcons: true
doNotCloseWindows: false
ignoreMinimized: false
accuracy: 20
fillGaps: true
fadeDuration: 1
showPanel: false
leftButtonWindow: 1
rightButtonWindow: 2
middleButtonWindow: 0
leftButtonDesktop: 2
middleButtonDesktop: 0
rightButtonDesktop: 0

highlightwindow:

blur:

contrast:

windowgeometry:
handlesMoves: true
handlesResizes: true

startupfeedback:
type: 1

screenedge:

kscreen:

If you use an Arch-based distro, then yes, you have to repackage kwin.

This is most likely a Dim Inactive effect issue.

Do you need help with repackaging kwin?

Created attachment 119553 [details]
PKGPUILD of kwin, modified

I think I repackaged it correctly 
[options=(debug strip)
-DCMAKE_BUILD_TYPE=debugfull
-DBUILD_TESTING=ON]
I installed both packages.
And now I have a usr/lib/debug/usr/bin/kwin_x11.debug
(and a few more .debugs in lib, sources in default directory as well)

Will the error-message-dialog display debug-symobls or should I have gdb attached to kwin all the time?
Can I somehow check whether the debug-symbols are attached correctly?
I have also been unable to reproduce the bug. Any tips?

Please repackage kwin with -DCMAKE_BUILD_TYPE=RelWithDebInfo.

> I have also been unable to reproduce the bug. Any tips?

Just wait. :-)

Dear Bug Submitter,

This bug has been in NEEDSINFO status with no change for at least
15 days. Please provide the requested information as soon as
possible and set the bug status as REPORTED. Due to regular bug
tracker maintenance, if the bug is still in NEEDSINFO status with
no change in 30 days the bug will be closed as RESOLVED > WORKSFORME
due to lack of needed information.

For more information about our bug triaging procedures please read the
wiki located here:
https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging

If you have already provided the requested information, please
mark the bug as REPORTED so that the KDE team knows that the bug is
ready to be confirmed.

Thank you for helping us make KDE software even better for everyone!

This bug has been in NEEDSINFO status with no change for at least
30 days. The bug is now closed as RESOLVED > WORKSFORME
due to lack of needed information.

For more information about our bug triaging procedures please read the
wiki located here:
https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging

Thank you for helping us make KDE software even better for everyone!