Hey, guys. Had the same problem. For me it was everything ok in **editor **viewport, but the issue appeared in a **packaged **version. I’ve double-checked - all level names were ok. Anyway, I changed my level names in all places it is used with the same value (just removed old ones and pasted the copied ones) and SOMEHOW it fixed that issue.
Yes, I know that this solution is kinda strange, but I guess it can be a bug in engine. Hope it will help someone