Hi Eytan,
I would like to confirm the following things:
1. Does it work from using Office 365 or occur recently?
2. Have you made some changes in Sharepoint Online recently?
Based on the current situation, I suggest you try to log in to the teamsite using another account which is granted appropriate permission to see if it works. Also, you can log in to the teamsite on different computers to check it.
Thanks,
Johnny Zhang