I have the same problem. The users have the correct registry keys. The users have the correct path. If you enable the debug log it states that an exception error has occurred. It was working perfectly, within a couple of weeks ago. I have multiple AVD hosts and a ton of users, it broke for all of them around the same time. My antivirus did not change, it did not happen right after a windows update push. I did not update FSlogix prior to it breaking. New Teams itself did have updates, however. I have reinstalled New Teams, the plug in, forced a re-write of the reg key, forced a register of the .dll . None of that worked. The only thing I've had temporary success with, is if you completely uninstall the Outlook Teams Meeting plug in "manually", as the logged in user, which requires granting them a highly privileged role (which is unrealistic), then "manually" reinstall the plug in, with the MSI - (because using the silent install with the correct parameters no longer works either). Then remove the elevated privs for the user and relaunch everything.
Then the plug in will work again temporarily. Until the user roams to another host. then it breaks again until those steps are repeated. They are using FSlogix as the profile roaming solution.
I cannot stress enough that this was working perfectly in production for months. It is super frustrating. The users must use legacy outlook for a specific reason, and right now they can only schedule meetings from 'new outlook' and Teams.
