Hi Roland,
Thank you for connecting with us in Microsoft community.
I was consulting your case with my team members, and I would like to share more specific information and direction with you.
According to your provided information and error, initially I have found one official document which is discussing several points for SSL certificate Certificate warning from AD FS when sign in to Microsoft 365, Azure, or Intune - Microsoft 365 | Microsoft Learn, initially You may take a look if your certain part of scenario is connected with this official document.
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Furthermore, if your concern is not matched with above provided information, according to your provided description, seems like your concern associated is also with AAD (Azure Active Directory) environment, and actually, Microsoft has separate community channel resources where our related most valuable professional and community members can provide possible information for certain different support scope resources platform, so would you mind to contacting and placing your concern on our dedicated separate Microsoft Q&A community channel?
Actually, this community does not focus the SSL certificate or AAD environment related concern, but this community is mainly focusing pure Microsoft 365 exchange online license and email flow related scenario.
However, there is an another specific channel for your mentioned concern where you may get some possible information for certain specific resources and platform from our related community members.
I would really appreciate your precious time. Thank you again for your kind cooperation.
Have a good day.
Sincerely
Vaidya Darpan