Durst L1200 column lock not working

Outer knob is rotating freely, the bolt is pushed in, I am not able to lock the enlarger in the column. The spring is operational, inner knob to move the enlarger also works...

Any ideas what's behind it?

Here it is - that is why the locking does not work - element is broken - does anyone has service manual of that thing? Maybe I can try to find the element when I will have any reference. Or maybe it can be glued?

You'll find a service manual for the Pictochrom and Pictograph at http://durst.loremi.com/ and those are just L1200 enlargers with some fancy technology added on, so the part of the manual that covers the carrier and its mechanisms is relevant.

I have no idea if parts are still available, but if not, you might be able to use the manual to help you get the broken part out, and find a metalworking shop where they can make you a replacement.