MADHOUSE, It’s known issue appeared in 4.16.
There is a patch already, hope it will be add to the new Unreal Engine.
For now as a workaround delete all Replicated variables and Network calls from your level blueprints.
Reason is that network objects (your level script is one of them, if contains replication) have different destroy rules, so their old versions are still in memory after level reload. So their blueprint wasn’t regenerated and still reffers to old objects.