Sorry I missed.
If you click start and then type credential manager is your gmail account listed under either category, Web or Windows.
Again you may see if as a Microsoft Account?
Have we discussed creating a test "Local" account on computer and seeing if it is allowed to sign in?
Did you ever find a solution? Still having the same problem. I managed to link my gmail-Account with the Microsoft WebApplication but the Calendar-App is still not syncing so far
I had the exact same issue and I also found the solution, so I figured I'd share and save someone else the headache.
TL;DR - Some conflict with one of the settings in "O&O ShutUp10++" broke it.
For those unfamiliar "O&O ShutUp10++" is the popular privacy tool for Windows 10 meant to disable a lot of "telemetry" from being gathered about you and sent behind the scenes to Microsoft.
People usually set it up once and then forget about it, not even suspecting that it be the cause of some unrelated issue later down the line.
I haven't yet figured out which setting in particular caused the Mail app to malfunction. I simply disabled all of O&O's settings to see if it was the culprit, and it was. Gonna start re-enabling them gradually and see if I can isolate the specific one that caused the issue and leave it turned off.
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