Switchboard is unable to launch the Multi-User Slate Server in launcher builds. This is because the default install location has "Program Files and "Epic Games" in the path (aka folders with spaces in them) and Switchboard is not quoting the executable path. We likely did not notice this previously because our local builds from Perforce have workspace paths without spaces in them.
This needs to be fixed in the 5.1.1 patch because it blocks usage of the Multi-User Slate Server, which is the current default in Switchboard (aka it will not work by default and the user will need to change the setting to fall back to the non-UI MU Server).
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-174166 in the post.