Thank you for your responses. I already maged to unlock shutter/advance lever.. Real issue is with one of the curtain or shutter blades. One of the blades does not come back to the original position. It is not horizontal like te other blades, and that creates gap in a curtain. It happens maybe in 50 percent of cases. Sometimes it returns as it should..
.. I already sent a quote request to one repair shop and probably will send one or two more.. As i wrote I do not feel comfortable trying to fix it by myself.. Hope quote will not be ove te roof.