"Broken MIME parts for mail"

[Poliman - Serwis]

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

Oct 16 10:03:22 s1 dovecot: imap(artur at 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 at 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 at 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 at 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 at 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 at 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?
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