After last weekend our Microsoft managed Tier 1 system stopped working unexpectedly. We would get "503 Service unavailable" error when trying to access the system via web browser. We hadn't changed anything in the system for 2 months.

Restart didn't help, so I checked the event logs. There I saw multiple times this error message: "A fatal error occurred while creating a TLS client credential. The internal error state is 10013."

Clearly something was changed related to TLS and it was not done by us. I contacted Microsoft support and received this answer: 

"The issue is caused by Azure security requesting TLS1.2 , while some of our requests we made from the DEV environment need to have TLS 1.1 and TLS 1.0 - we are working to release a fix for PU29, 30, 31, but until this is released, I have raised an internal ticket for us to fix it manually. "

Microsoft managed to fix it for us and the system worked again correctly. I'm sure that everyone who has a MS managed Tier 1 system which they use for other than build purposes will also face the same issue, so I wanted to share the solution with you.