The experimental version has a new map format. You either tried to load project without converting it to the new format or tried to open a converted map in the normal version.
Map was originally created in an experimental version. And kept only in it. I tried to open a map from the experimental version in the normal one - it opened. But in the experimental - refuses to open.
Initially, it stopped opening in the experimental vesrion. In the usual opened in order to see if there will be the same error or not.
Nothing meaningful in the error log? I can't make it out myself.