I've seen this before, only rarely however.Something I tried to which seems to have helped a little to ensure the time between mailchecks is kept to at least 5 mins. Having the system check email again before the one finished is bound to cause some sort of trouble.Someone mentioned a fix they have found to work for them, it is the currently the last post mentioned in this issue on github: https://github.com/osTicket/osTicket/issues/4011