Since this is for a business, rather than offer a 'fix', I offer a strategy. - I would revert to a version that is stable for your environment - set all copies to NOT update automatically. - if thunderbird is meeting business needs, do NOT update for six months - at six month intervals (or whatever works for you) review release notes of latest update and update one PC. use the update for a minimum of a week on the PC, doing all normal workload tasks. Then, once you're comfortable, update the other PCs.

In essence, treat Thunderbird as any other business tool: don't change just to change. update when the update keeps you eligible for support and when the update adds useful features. For example, 128 is the current release and 115 was the prior release. Requesting help for any version prior to 115 may go unheeded or you will be asked to upgrade immediately. There, just my suggestion, based on my own experience with business software.  :)

What is the full version number?

• Start Windows in safe mode with networking enabled https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode
• Still in Windows safe mode, start Thunderbird in Troubleshoot mode https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird

Does the problem change?