Hi,
we have the same problem in our company. Since a few days people cannot sign in the Teams-App on iOS-devices with their Office365-guest-accounts. On Safari it is possible to sign in.
The same error "ChidchanokMonroek-9397" mentioned.
We did nearly everything. Kicked the guests out of the organisation and joinend them again. Some user even reset their iPads, but it does not work.
This just referes guest-accounts on iOS-devices (iPad, iPhone). On Desktop-Devices (Windows, MAC) or in webbrowser there is no problem.
The guest-accounts are all invitend from licensed-account of our organisation.
Best regards
Malte