If you create your own deck rule set in the DeckRuleSet folder.
Then use it and later rename it, you will get a problem when starting up again.

For some reason it seems like HearthRanger tries to parse all files that are included in the HearthRanger - Bot for Hearthstone.tsk file.
Not sure why you have to load the files in the old tasks, seems like this is the bug. Because that is the problem.

In order to solve this I had to rename the file within the tsk file to get it working again.