Hi Kevin, thanks for your guidance looking at this. I found I did need to enable to IMAP logs on the mail server, and was able to collect some logs.
From the osTicket v1.18 server, the following connection appears in the logs on the mail server when trying to add the IMAP account in osTicket:
sIp: [mail server]:993
cIp: [osticket server]:53180
So, all it appears to do is try to open a session. In contrast, if I perform the same action on the osTicket v1.16 server, I get logs for a similar Open Session command, but additional "capability", "login", and "CloseSession" commands. The v1.18 does not appear to be attempting any of these commands.
The fact that rqsize and rpsize are both 0 seems odd, but I'm not sure what is normal for this.
Hopefully that provides some helpful information for further steps. I will try to look into the PHP logs, though I'm not familiar with what to do there. I'll update if I find anything.