Ok, folks!
Here the solution. Short one, simple one, but it did work for me.
The rule, to avoid this situation is - toolbar must be the last form added into form set. No form added after toolbar, it crashes. I've deleted toolbar in my formset, saved it and then added my toolbar again. Now it works, can't complain. Seems like bug...
XO4Y BCE 3HATb,YMETb,IMETb!