I had this intermittently on the Zaf, same engine and gearbox as I understand it, it was a failed master cylinder in my case. In my case the failure progressed to it being hard to select a gear and ultimately the clutch not properly disengaging at all.
The part was about £25 on ebay for one in a genuine GM holographed baggy.
Changing it on the zaf involved wipers and scuttle panel off, remove a locking tab and take off the tube off where it connects down to the slave. There is also a second smaller tube that just clips into the reservoir. Then on the inside, take off a lower trim panel, remove the clip and pin that secures the slave cylinder to the pedal and undo a couple of 10mm nuts. Swap old for new and assembly is the reverse of removal as they say. Some people on forums had issues bleeding the system afterwards, but I found that part ok to do.
Truth be told I found working inside the car a bit of an ar$e to do as its up under the dash and I only had a basic toolkit. Also, I'm too fat for lying upside in the footwell of a car.
I would imagine its going to be 1hr labour max at a garage so <£100 to get someone to sort it for you.
EDIT: Just seen you're in Finland, so I have no idea how much it would cost at a garage