I also get these errors quite regularly. In my latest case, the message read “IlleagalArgumentException - true”. It looks like gephi files with multiple workspaces are more prone to these errors, but I am not 100% sure this is the case.
Anyway, it makes working with gephi a bit like Russian Roulette. After saving the result of your work, you might or might not be able to open it again. :/
Is there any way to fix these files? Would it help to provide a file for inspection?
I am running Gephi 0.8.2 beta on Ubuntu, but it looks like this does not depend on the operating system (https://forum.gephi.org/viewtopic.php?f=4&t=2837
describes the issue on Win8).