I have many lines like below in error.log file:

Oct 16 10:03:22 s1 dovecot: imap(artur@firmastanaszek.pl): Error: unlink(/var/vmail/firmastanaszek.pl/artur/Maildir/dovecot.index.cache) failed: No such file or directory (in mail-cache.c:28) Oct 16 10:03:22 s1 dovecot: imap(artur@firmastanaszek.pl): Error: Corrupted index cache file /var/vmail/firmastanaszek.pl/artur/Maildir/dovecot.index.cache: Broken MIME parts for mail UID 1019 in mailbox INBOX: Cached MIME parts don't match message during parsing: Cached header size mismatch (parts=) Oct 16 10:03:23 s1 dovecot: imap(artur@firmastanaszek.pl): Error: unlink(/var/vmail/firmastanaszek.pl/artur/Maildir/dovecot.index.cache) failed: No such file or directory (in mail-cache.c:28) Oct 16 10:03:23 s1 dovecot: imap(artur@firmastanaszek.pl): Error: Corrupted index cache file /var/vmail/firmastanaszek.pl/artur/Maildir/dovecot.index.cache: Broken MIME parts for mail UID 1052 in mailbox INBOX: Cached MIME parts don't match message during parsing: Cached header size mismatch (parts=) Oct 16 10:03:23 s1 dovecot: imap(artur@firmastanaszek.pl): Error: unlink(/var/vmail/firmastanaszek.pl/artur/Maildir/dovecot.index.cache) failed: No such file or directory (in mail-cache.c:28) Oct 16 10:03:23 s1 dovecot: imap(artur@firmastanaszek.pl): Error: Corrupted index cache file /var/vmail/firmastanaszek.pl/artur/Maildir/dovecot.index.cache: Broken MIME parts for mail UID 1098 in mailbox INBOX: Cached MIME parts don't match message during parsing: Cached header size mismatch (parts=)

I didn't do anything with Dovecot neither update in last time nor move files or something. Does anybody could tell me what above means and how to fix it?