Just experienced this issue today. Note that there was also a crash of kded, which might have caused the issue. With all that said, this "akonadi_migration_agent" popup usually shows on every login and goes away after about 2-3 seconds.

I still have this. October 2024.
At first glance I would ask: why isn't there the possibility to "never show again" or something like that, but still, that would be asking the wrong question.
The right question would rather be: who at all would like to be notified about this? In what way is the fact that the akonadi_migration_agent completed it's task important to me???

(In reply to PK from comment #2)
> I still have this. October 2024.
> At first glance I would ask: why isn't there the possibility to "never show
> again" or something like that, but still, that would be asking the wrong
> question.
> The right question would rather be: who at all would like to be notified
> about this? In what way is the fact that the akonadi_migration_agent
> completed it's task important to me???

I found the only "fix" for this is to get rid of all things KDE. The Akondi database has been a tragedy from the beginning. I've been in it roughly 40 yeas now and started with SuSE when it came in two boxes: one with floppies, the other with printed documentation.

I installed NextCloud on an old i7 gen4 or gen6 in my office.
https://nextcloud.com/
I installed BetterBird on my machines.
https://www.betterbird.eu/

Also installed a 16TB NAS (which doesn't sound as big as it used to).

Now, Manjaro, Windows 10, Linux Mint, openSuSE Leap, all share the same documents, contacts, etc. None of them run any KDE packages and none have this problem.

I am observing this issue regularly after updates to KDE Plasma packages on Arch Linux. On next reboot, SDDM typically takes much longer than usual to show up. After successful login, I observe a black screen with blinking cursor. If I wait long enough, finally the KDE Plasma desktop will show up with the akonadi_migration_agent "In progress" dialogue spinning. With that dialogue active, KDE Plasma seems to have also trouble to cleanly logout the session, shutdown, or reboot the computer. If one tries to work in that session, I observed freezes of applications after a short time, which normally run without issues, namely Firefox, Thunderbird, and Libreoffice.

In the past rebooting typically helped to make the issue go away until the next update of KDE Plasma packages. However today, the issue would persist even after several reboots. Only removing ~/.cache in a console session before logging in via SDDM into KDE Plasma (Wayland session) helped. KDE Plasma is at version 6.3.2 currently on Arch Linux.

(In reply to Torsten Maehne from comment #4)
> I am observing this issue regularly after updates to KDE Plasma packages on
> Arch Linux. On next reboot, SDDM typically takes much longer than usual to
> show up. After successful login, I observe a black screen with blinking
> cursor. If I wait long enough, finally the KDE Plasma desktop will show up
> with the akonadi_migration_agent "In progress" dialogue spinning. With that
> dialogue active, KDE Plasma seems to have also trouble to cleanly logout the
> session, shutdown, or reboot the computer. If one tries to work in that
> session, I observed freezes of applications after a short time, which
> normally run without issues, namely Firefox, Thunderbird, and Libreoffice.
> 
> In the past rebooting typically helped to make the issue go away until the
> next update of KDE Plasma packages. However today, the issue would persist
> even after several reboots. Only removing ~/.cache in a console session
> before logging in via SDDM into KDE Plasma (Wayland session) helped. KDE
> Plasma is at version 6.3.2 currently on Arch Linux.

Having come up on real computers with real operating systems where actual Software Engineering (not Agile) and physical QA teams (not TDD) were used, I am always mortified at the roughly 99% of PC applications that can't bother to clear their disk cache on startup. Adding insult to injury, the format of the files changes between updates, new software is never tested with the old cache format and doesn't check for it, then all sorts of gastric intestinal application issues happen after an update is pushed out.

If the software development process can't be fixed, at least fix the update process so it closes the application and nukes the cache before applying the update.

Cache is meant only for the current running instance, not for saving settings between runs.

It started happening on my Fedora KDE installation after the recent update. It is just stuck forever.

Operating System: Fedora Linux 41
KDE Plasma Version: 6.3.2
KDE Frameworks Version: 6.11.0
Qt Version: 6.8.2
Kernel Version: 6.13.5-200.fc41.x86_64 (64-bit)
Graphics Platform: Wayland
Processors: 32 × AMD Ryzen 9 5950X 16-Core Processor
Memory: 62.7 GiB of RAM
Graphics Processor: AMD Radeon RX 6700 XT
Manufacturer: Micro-Star International Co., Ltd.
Product Name: MS-7D52
System Version: 1.0

(In reply to roland from comment #5)
> (In reply to Torsten Maehne from comment #4)
> > I am observing this issue regularly after updates to KDE Plasma packages on
> > Arch Linux. On next reboot, SDDM typically takes much longer than usual to
> > show up. After successful login, I observe a black screen with blinking
> > cursor. If I wait long enough, finally the KDE Plasma desktop will show up
> > with the akonadi_migration_agent "In progress" dialogue spinning. With that
> > dialogue active, KDE Plasma seems to have also trouble to cleanly logout the
> > session, shutdown, or reboot the computer. If one tries to work in that
> > session, I observed freezes of applications after a short time, which
> > normally run without issues, namely Firefox, Thunderbird, and Libreoffice.
> > 
> > In the past rebooting typically helped to make the issue go away until the
> > next update of KDE Plasma packages. However today, the issue would persist
> > even after several reboots. Only removing ~/.cache in a console session
> > before logging in via SDDM into KDE Plasma (Wayland session) helped. KDE
> > Plasma is at version 6.3.2 currently on Arch Linux.
> 
> Having come up on real computers with real operating systems where actual
> Software Engineering (not Agile) and physical QA teams (not TDD) were used,
> I am always mortified at the roughly 99% of PC applications that can't
> bother to clear their disk cache on startup. Adding insult to injury, the
> format of the files changes between updates, new software is never tested
> with the old cache format and doesn't check for it, then all sorts of
> gastric intestinal application issues happen after an update is pushed out.
> 
> If the software development process can't be fixed, at least fix the update
> process so it closes the application and nukes the cache before applying the
> update.
> 
> Cache is meant only for the current running instance, not for saving
> settings between runs.

Deleting everything in ~/.cache works for me