That’s unexpected. Can you try to recreate an account and gather logs from the mailer daemon ?
Edit or create ~/.config/QtProject/Messageserver.conf, so it contains:
[FileLog]
Enabled=1
Path=~/qmf.log
(put another path if you prefer, and you can keep other entries if they already exist)
Then restart the mailer daemon systemctl --user restart messageserver5 and try to create an account. Go until the failure. Then, look at the ~/qmf.log log, PM it to me or post it here.
To stop file logging of the mailer daemon, put the Enabled entry to 0 and restart the mailer daemon.