I am having the same issue, only I am trying to use 1 witness server for 2 mirrored databases that reside on different servers. For example, I have db1 on server(A) being mirrored to server(B) using witness server(1), and db2 on server(C) mirrored to server(D) but I cannot get it High Safety using witness server (1) on the same port 5022.

I get error message: The ALTER DATABASE command could not be sent to the remote server instance 'TCP://witness server(1)'. The database mirroring configuration was not changed. Verify that the server is connected, and try again.

Mirroring is already up and working on the first db1 using witness1, and mirroring also works on the second set, however the second db on server(2) is unable to connect to the witness. The witness shows this error: Message

Database Mirroring login attempt failed with error: 'Connection handshake failed. The certificate used by the peer is invalid due to the following reason: Certificate not found. State 89.' Client x.x.x.x

I copied the original witness server cert over to both db2 primary/secondary servers. All 3 certs are on each server.

Not sure why this is happening. I am using SQL authentication. I can telnet to every port too. Microsoft Documentation of the witness server appears to suggest that you can use a single witness for multi pair mirroring, but doesn't tell you anything about how to do it.

Any help would be fantastic! (Using SQL Server 2014 Std on Windows 2012R2 x64 machines in AWS)