AADSTS500116 error without an apparent cause #432

Hello everyone,

We have a few websites connectin Outlook to FluentSMTP. On several of them we're getting the error AADSTS500116: The reply uri specified in the request is not a valid URL. It states that "specified in the request does not match the redirect URIs configured for the application" which is a false statement unless the callback is using a different URI than the one it defined.

Some context:
All of our websites use WPML, and are bilingual with a .pt and a .es version. In the authentication on Azure, I haded both the URI from FluentSMTP, and then added a .es version of it as well, as I was having the problem already - it didn't solve it.
All of the sites where we're having the problem are like that (www.xxx.pt / www.xxx.es), while our main site, that doesn't have the same problem, is .com on the portuguese version and .com/es on the spanish version (www.yyy.com / www.yyy.com.es).

All the websites have a e-mail account, with aliases. The main site has an account on Outlook/Microsoft 365, the other ones were hosted on Gmail, but now we exported them to our Outlook account as shared e-mail accounts, so we can manage all the e-mails on one tab.

The problem is not happening constantly, but it's happening with enough frequency that I have to go and check the failed e-mails here and then, missing the point of having an automated e-mail system.

As the only differences between the accounts are the ones stated above. Is it a WPML problem or is it something about the Outlook managing shared e-mail accounts and some incompatibility with the FluentSMTP?