SUMMARY
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After having installed NeoChat using the procedure for flathub, configuring my account and the interface (dark theme), I've closed it and on all the following startups NeoChat keeped crashing because of an assertion error.

STEPS TO REPRODUCE
1. install NeoChat using the "download on flathub" button here and follow the procedure https://apps.kde.org/neochat/ (stable version)
2. connect my account on matrix.org
3. configure the theme to be dark mode 
4. close NeoChat
5. relaunch NeoChat
6. it crashes during startup

I'm honestly not convinced at all that step 2 and 3 having any impact here. According to the log it's very possible that a specific configuration for a specific room is the source of the problem but I don't know how to get access to more information.

Here is the log observed in the shell:

Reading access token from the keychain for "@Bram_:matrix.org"
quotient.main: The library is built with end-to-end encryption (E2EE)
quotient.e2ee: Successfully loaded pickling key from keychain
quotient.e2ee: 65536 bytes of secure heap initialised
quotient.e2ee: Allocated 128 bytes of secure heap (requested 128 bytes), 128 / 65536 bytes of secure heap used in total
quotient.e2ee: The fixed buffer source is shared; assuming that the caller is responsible for securely clearing other copies
quotient.e2ee: Allocated 256 bytes of secure heap (requested 160 bytes), 384 / 65536 bytes of secure heap used in total
quotient.e2ee: Deallocated 256 bytes, 128 / 65536 bytes of secure heap used
quotient.main: Malformed userId: "@.m.rule.is_user_mention"
quotient.main: Malformed userId: "@.m.rule.is_room_mention"
quotient.main: Malformed userId: "@.m.rule.suppress_edits"
quotient.main: Malformed userId: "@.m.rule.contains_user_name"
Qt Quick Layouts: Detected recursive rearrange. Aborting after two iterations.
Qt Quick Layouts: Detected recursive rearrange. Aborting after two iterations.
QQmlComponent: Cannot create new component instance before completing the previous
qt.gui.icc: fromIccProfile: failed minimal tag size sanity
libpng warning: iCCP: known incorrect sRGB profile
qrc:/EmojiDialog.qml:57: TypeError: Cannot read property 'width' of null
qrc:/EmojiPicker.qml:25: TypeError: Value is undefined and could not be converted to an object
ASSERT: "isValidFor(room, oldMarker)" in file /run/build/libquotient/Quotient/eventstats.cpp, line 60

OBSERVED RESULT

NeoChat crashes on startup.

EXPECTED RESULT

No crash.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Ubuntu LTS jammy
(available in About System)
KDE Application Platform (org.kde.Platform from flathub): 5.15-23.08

ADDITIONAL INFORMATION

I'm available if you need more information or if you want me to provide the content of some configuration files.

Kind regards,