Hi Slavek,
Apologies for the delay, it seems we’ve missed your post.
So your issue seems similar to this one:
Object reference not set to an instance of a object
Unfortunately we were unable to pinpoint what’s causing it 
Please have a look at that topic, perhaps you’ll find some suggestions helpful.
Other than that, if you do have some CMS/client/windowsEvent logs that could shed some light on this issue, we’d appreciate it.