[jb_s]

Azure stopped supporting TLS 1.1 recently as well.

If these are old systems, it could be this... if they're running .NET apps compiled against framework pre... 4.7? 4.8? TLS ain't gonna work...

[jiggawatts]

They would only be affected by the Azure TLS changes if they already used Azure APIs while on-prem, which would be unusual for an on-prem-to-cloud lift & shift.