The works shown below are recommended only for precision engineering savvy professionals (hobbyist?).
I decided to disassemble the switch - which due to its solid construction was fortunately not a problem.
I found that the surfaces (faces) of the optical fiber were very uneven. I decided to grind these areas plan. I used to grit sandpaper 1200 (in paintshops usual). In addition, I polished with "Paris Red" after (maybe not necessary).
After assembling all switch positions functioned smoothly.
The manufacturer I can only recommend to plan and polishing of the optical fiber in the production process with take. For lay people the unit (transmit and receive performance of opto-components depend) not to be used under certain circumstances as.