I had a very similar problem today, along with several error messages, trying to authenticate OneDrive to sync previously saved Vault data (by another device) with a newly installed Enpass Password Manager 6.6.0. This was downloaded from Microsoft Store and I did not have a previous version on my computer.
Lukasaz1999, your trick with IE did the job, very clever and thank you for posting that!
However, I should urge Enpass to investigate this occurrence because the dependency on IE is not justified today. This browser is being deprecated by Microsoft and replaced with Edge. I suspect this issue is related to the API in use because authentication seemed to stumble over that. Thanks for your attention to this matter.