it helps for me (dovecot 2.2.36 @ centos7):

find /path/to/user/maildir/ -type f -name dovecot.index* -delete

find /path/to/user/maildir/ -type f -name dovecot-uidlist -delete

#then re-create index by command:

doveadm force-resync -u "user@domain.com" '*'