Login failed. The login is from an untrusted domain and cannot be used with Windows authentication.

I encountered this problem today on a server running SCVMM 2012. I found a lot of things online suggesting a lot of different possible solutions, but none of them worked for me. Most of the possible solutions involved configuration errors with authentication, but this has worked fine for some time, and only today started giving this problem. Besides that, this server hosts the databases for a number of other apps, and none of those stopped working today, so after a quick check over the SQL Server settings I was left scratching my head.

After a long while of looking for this needle in the haystack, I lucked my way into the problem. What I found was that the connection string had changed. I checked this after checking the registry keys pointing to the SQL Server on the SCVMM box (which were fine). you'll find the REG_SZ at:

HKLM\SOFTWARE\Microsoft\Microsoft System Center Virtual Machine Manger Server\Settings\Sql\ConnectionString

A while back I upgraded a database server to a better hardware platform.  For various unrelated issues, I decided to go with a fresh start and build a new server from scratch rather than migrate it to the new hardware. In doing so, it was given a new name and coexisted with the old server for a short time.  Rather than the recommended method, I edited the registry on the SCVMM server to point it to the new database server (all settings remained the same except for the hostname) and after a reboot, everything came up as expected and worked fine.  What I found today is that the name of the server in the connection string had reverted back to the old server, or it always was and worked until something else recently changed. But either way, I changed the connection string, and we're back up and running.