I encountered a situation where the disk space was filled up during server operation. The 'LevelMeta.sav' file in my save folder became 0 bytes, but the 'Level.sav' file and the data in the 'Players' ...
Tried to open on all .sav files, the sav from the world as well as the one from players. Also tried from backup files. It simply would not open the GUI after selecting the .sav file.