UE4 Plugin (4.21) for resolving conflicting material IDs that cause errors in Swarm build
After placing in project>plugins, regenerating and stuff you should have new button on the toolbar called SwarmGuidFixer. Load that problematic level, click the button and then Save All.
thanks to: https://answers.unrealengine.com/questions/68053/lighting-build-failed-swarm-failed-to-kick-off.html