"Failed to connect DB , Failure to access database server"

[MoMohamad]

I have DB Server with OS win server 2012 R2 with SQL 2012 sp1 DB installed, this DB Server has two instances.

and i have two identical application server with arcmap 10.8 and win server 2019, both application server were connecting normally to DB Server suddenly one of them stopped connecting to one of DB Server instance and still connecting to other instance, an error appears "Failed to connect DB , Failure to access database server" 

Knowing that all Server in same network are pinging to each other, no firewall no antivirus. SQL native clients are installed also i installed ODBC, we checked all security policies, nothing worked.

also i checked DB server all services are running, nothing seems unnormal.

All Sql services are running and connecting normally to other clients PCs, except one server. 

I did everything, i don't know what's causing this!

[JoshuaBixby]

Start by taking ArcGIS or any other server software out of the equation and focus on the connection SQL connection.  Can you make SQLCMD connections to both database instances from both servers?

[MoMohamad]

when i try to test the connection from the server which has the problem 

 

[JoshuaBixby]

This confirms my suspicions that it isn't an Esri/ArcGIS issue.  Although someone on GeoNet may have run into this situation and have suggestions on resolving the error, I suggest you search the web for solutions to your SQL Server connection issue.  A quick search on SQL Server Error 18 related to SSL Security provides lots of hits to start researching.