I ran into the same issue on UE5.2.0 release. Seems specific to IKRig and animations when running in stand-alone mode.
This assertion (failed) was not present when I last tested my game in standalone mode using UE 5.1.1, and quite possibly UE 5.2.0 preview 2 as well.
Extra info:
My game is 100% blueprint based, no c++ code (yet) so trying to work around this assertion failing seems like an impossible task unless I edit a copy of UE 5.2.0’s source code and block out that assertion check.
I’m assuming this was done as part of an anti-cheat system for Fortnite but somehow ended up as part of the general 5.2.0 release code?