I am using SQL Server 2008 R2. It is working fine. But recently, I have changed my hosting server and I came to know that they have installed SQL Server 2
SQL Server 2008 R2
SQL Server 2
It's very old problem with cashed content. MS planning to remove diagrams from SSMS, so they don't care about this. Anyway, solution exists.
Just close Diagrams tab and open it again. Works with SSMS 18.2.