Are you storing a reference to the “previous level” at runtime ?
If so, this could be causing PIE-specific issues because as you can see in the error message, PIE renames the loaded level into UEDPIE_#_LevelName. So if you store that in a variable, then you move to next level, then you try to come back to previous level, it doesn’t exist anymore.
Simplest way I can think of right now is to branch based on “Is Play In Editor” node and figure out how to cut the UEDPIE part from level name.