[Dovecot] Error that I don't understand

[Andre]

I’ve noticed lines like this in error log:

IMAP(username at cubeholding.com): Error: read(mail, uid=58284) failed: Is a directory

I’ve looked at sources and found this message in src/lib-storage/index/index-mail.c, function static int index_mail_stream_check_failure(struct index_mail *mail) but I don’t understand where it come from, I understand it is something about indexes, anyone can explain me more, please?

Dovecot 1.2.12

TIA
Andrea