Thunderbird losing connection to IPSWITCH mail server

I am running an environment using IPSWITCH MAIL SERVER (for 15 years btw). I am running multiple workstations (some on physical some virtual) all on Windows 10. I set up Thunderbird (latest version 64 bit) and create my users per workstation. On each workstation the E-mail sets up and works using 993/465 - SSL/TLS - ENCRYPTED PASSWORD. After set up Thunderbird performs perfectly. On EVERY workstation within one or two days the accounts have to be deleted and re-created USING ALL OF THE EXACT SAME PARAMETERS and Thunderbird works again (for one to two days) and then I need to repeat the SAME exercise. About two dozen repeats now. No changes in the network environment. No changes on the mail server. Thunderbird has given me yeoman's service in two firms for twenty five years now and this has just started this year since February. Any ideas anyone?