[Dovecot] Canot move messages out of my inbox

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I'm using thunderbird on top of Maildir, dovecot latest.
For some weeks i cannot move messages out of the inbox folder anymore.

Looking at the logfile, i see

Error: child 19591 (imap) killed with signal 1

which by now is the only error i can see.
I have already compared permissions on working folders with the inbox,
all are the same (rwx, owner, owner).

Any ideas?
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