Hello Blaz did you get to solve this problem? i have the same problem and i dont know how to solve it, please help me...
Having the same problem with ArcGIS 10.2.2 HEC-GEOHMS 10.2 windows 10 pro. Latest .NET Framework is also installed. All target locations are in local drive C: and paths containing only english characters. I have tried other disks/paths with no result
Sometimes I save and close Arc Map then reboot and this fixes the problem.
I've often wondered if Arc Map has trouble with the "undo" list. That is, if you've made a lot of changes with saving/reopening the .mxd, maybe it can't handle that and needs to have a "clean slate" to proceed. Only a theory...