On 28 Mar 2019, at 10.15, Arkadiusz Miśkiewicz <arekm@maven.pl> wrote:

 error = 0x55e3e2b40ac0 "Fixed index file
/var/mail/piast_efaktury/dovecot.index: log_file_seq 13 -> 15",
 nodiskspace = true,

This was one of the things I was first wondering, but I'm not sure why it's not logging an error. Anyway, you're using filesystem quota? And this index is large enough that trying to rewrite it brings the user over quota?