Msg 21, Level 21, State 1, Line 1 Warning: Fatal error 9001 We recently moved to a SQL Server that's on a VPS and every now and then we get a 9001 error when trying to do an update.

Rebooting the server fixes the problem.

I changed the maximum server memory (in MB) to 2000 because it's a 4GB VPS.

But I just don't know what's causing the error.
That error means that the log for your database (or for tempdb) is not avaialble for some reason.  That's a serious problem that is usually fixed by restarting the server.

If the problem recurs, it could be many things having to do with problems with your storage system.  One possible thing to check is to see if your database's log file (or tempdb's log file) is running out of space (not sure if that can cause this error or not).