It’s a old post, but I just had this issue, in my case I wasn’t being able to even enter the project, this is how I fixed:
I deleted the “DefaultEngine.ini” from the sub-folder “Config” on the project folder.
After that my project launched, and I saw that what was causing the error was that I had setup some watch points on the animblue print of some characters placed on the start map, after I removed the watchpoints from the nodes, the map was working fine, I closed the project, restored the “DefaultEngine.ini” and everything is peachy.
To avoid this problem I guess you better not debug anything from the starting map.