"Object reference not set to an instance of an object" when saving a file with a segment with comment

Hello,

I need to add yet another item to the a quite long list of occurrences of "Object reference not set to an instance of an object" error message. In my case, it shows up every time I'm trying to save a project file with a segment commented (tested with different types of comment FYI, Warning, Error - all the same).

See the log file for more info.

Any ideas anybody?

Thank you for help in advance, it will be greatly appreciated.

Jonáš.