I'm working on a customer tenant with my AAD credentials. The customer invited me into a Office 365 Group as a Guest. The access is working fine and I'm also getting the email. At the moment we are working on a really large document with multiple people. I work onsite and most of the times I open the document (in Word 2016 current branch on my AAD joined Windows 1703 device) and work all day. Sometimes the document is open for more than 8 hours.
Yesterday I received at the end of the day an error message and word was not able to upload the document. I could not figure out how to fix this problem. Upload center showed a permission error and allowed me to do a sign-in. After a successful sign-in the document was still stuck in upload center. I opened the SharePoint site (of teh group) directly but I also received an access denied message, also I was working on the document and the site all day.