Can you try to follow imap LOG when you try to login?
You’ll see this kind of log if all it’s good, else you’ll see some errors:
Feb 12 23:46:00 mail dovecot: imap-login: Login: user=<admin>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=20018, secured, session=<FPd7MLqBFrh/AAAB>
Feb 12 23:46:00 mail dovecot: imap(admin@domain.it): Logged out in=87 out=849
Feb 12 23:46:00 mail dovecot: imap-login: Login: user=<segreteria>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=20023, secured, session=<ioV8MLqBGLh/AAAB>
Feb 12 23:46:00 mail dovecot: imap(segreteria@domain.it): Logged out in=87 out=857
Feb 12 23:46:00 mail dovecot: imap-login: Login: user=<talenti>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=20027, secured, session=<BDJ9MLqBGrh/AAAB>
Feb 12 23:46:00 mail dovecot: imap(talenti@domain.it): Logged out in=87 out=881
Feb 12 23:46:03 mail dovecot: imap-login: Login: user=<admin@domain.it>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=20035, secured, session=<VTGbMLqBHLh/AAAB>
Feb 12 23:46:03 mail dovecot: imap(admin@domain.it): Logged out in=82 out=767
Feb 12 23:46:04 mail dovecot: imap-login: Login: user=<admin@domain.it>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=20048, secured, session=<VbKpMLqBILh/AAAB>
Feb 12 23:46:04 mail dovecot: imap(admin@domain.it): Logged out in=44 out=628
Feb 12 23:46:05 mail dovecot: imap-login: Login: user=<admin@domain.it>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=20057, secured, session=<LFW9MLqBJLh/AAAB>
Feb 12 23:46:05 mail dovecot: imap-login: Login: user=<admin@domain.it>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=20058, secured, session=<pHi9MLqBKLh/AAAB>
Feb 12 23:46:05 mail dovecot: imap(admin@domain.it): Logged out in=233 out=1062
Feb 12 23:46:06 mail dovecot: imap(admin@domain.it): Logged out in=296 out=2801
And check also /var/log/maillog and /var/log/messages.
Thank you.