Email fetching is intermittent -- Auto Cron fires but responses to tickets are not appended. Set to 'Do Nothing' for fetched emails.

Never rely on Auto Cron -- set up a proper Windows Scheduled Task or Linux cron job instead (see https://docs.osticket.com/en/latest/GettingStarted/POP3-IMAPSettings.html). Change the Fetched Emails setting from 'Do Nothing' to 'Archive - Move to Folder' and set an archive directory. BEFORE changing this setting, move all already-processed mail out of the INBOX or the system will reprocess every historical email. The root cause here was a full/overloaded inbox combined with a malformed email -- clearing the inbox and archiving processed mail resolved it.