SUMMARY
***
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***
Replying to the last email in a long thread (about >10 emails) becomes impossible: at first, there si long delay betwen typing and the actual input in the composer window. After some time, the window freezes, typing-in is no longer possible and a pop-up window suggests to terminate Kmail2. Teminating causes Kmail2 eventually to crash. Restarting Kmail2 takes quite long, then the last message window pops up agin and must be killed to work with Kmail2 again, but replaying to the last email is not possible. 

STEPS TO REPRODUCE
1. Produce rather long threaded message list (about more than 10 replies emails).
2. Try to reply to last message.
3. Typing in answering window heavily delayed, finally Kmail freezes, message pops up after about 1 minute, suggests to terminate Kmail2 - no more action possible part from terminating.
4. Terminate Kmail2.
5. Restart Kmail2.

OBSERVED RESULT
Kmail freezes, message pops up after about 1 minute, suggests to terminate Kmail2, crashes after termination, starting Kmail again gives a long delay, last message window pops up again and must be killed to go on. 

EXPECTED RESULT
Simply reply to last message!

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: KDE Frameworks 5.112.0
(available in About System)
KDE Plasma Version: 5.27.9
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
Was already claimed yers ago - still not solved!!