I am seeing the same error at random times. Sometimes several days no error, but then 2-3 in a row - one per minute.
I do not see authentication error in mail logs (unless I am looking at a wrong place but I have checked several ones)
Error: Can not authenticate to POP3 server: POP3 connection broken in response
6 consecutive errors. Maximum of 5 allowed
This could be connection issues related to the mail server. Next delayed login attempt in aprox. 10 minutes
I am using latest 1.15.4 version, PHP is 7.4 running on ubuntu 20.04 with Postfix and Dovecot where pop3+SSL on port 995 is running.
I have no issues sending nor receiving emails , just getting this notification about error from OSTicket.