Same issue here.
What happened is that Microsoft disabled legacy protocols (like POP3) for your entire tenant (all mailboxes). You need to enable back the "basic authentication" support to use POP3, IMAP and SMTP again. Please follow: https://techcommunity.microsoft.com/t5/exchange-team-blog/basic-authentication-and-exchange-online-june-2021-update/ba-p/2454827
In few months, for continue to use M365 mailboxes for tickets collection, having osTicket support for modern authentication will be mandatory as Microsoft will end the basic one definitely.
WARNING: I'm using the old 1.9.x version and deleting the mail account entry in osTicket has been necessary to bypass the "too many logins" error. It seems that, in some way, the mail account entry in osTicket remained "stuck" on that error.