Hi Don,
I would start by viewing Azure Active Directory's sign-in logs:\
Sign-in logs in Azure Active Directory
https://learn.microsoft.com/en-us/azure/active-directory/reports-monitoring/concept-sign-ins
Also, when they receive the error, is this a Microsoft 365 error message? Have you tried to expand more details to find the exact error message?
Sign-in error codes
If this is helpful please accept answer.