Recently I bought some used actuators (local store for used electronics). One of them was Roh'lix type:
Roh'Lix (http://www.zero-max.com/products/rohlix/rohlixmain.asp)

I noticed no backlash. I cannot move the carriage by hand (but it's not hard to turn the shaft/"screw" by hand and move the carriage that way).
I haven't much information about these. Can anybody help me - especially problems, maintenance etc. ?

I have used them in a CNC back gauge installation, The tension has to be monitored every so often or slippage may occur, generally they work well. Most applications that use them however, use linear scales on the motion part rather than an encoder on the Rohlix shaft.
Although I think if you do not have a great degree of back pressure, then the encoder on the shaft should work.
There are quite a few up sides, no lubrication, no backlash etc.
Al.