I’ve heard from another dev on my team that invalid IL errors may occur in the case that [Server] is on a method that returns a value in 5.1.1f1 (64-bit, PC). His workaround is to return the value as an out argument, although I had just been commenting out the [Server] attributes for these cases.
Oops, never mind, it appears to be fixed in 5.1.1f1