Hermann, That is useful.
I have both the lenses.
I had already disassembled the Sony E 16mm.
After a long hard thought, this is the best explanation I could come up with. The E 16 travelled with me to India 2 summers ago. Was in ~35 deg celcius for just about 2 or 3 days. That did the damage to the plastic.
A few summers earlier, the Olympus 17/2.8 was in Barcelona at about ~40 deg Celcius.
Both the lenses have pretty much the same design and material construction as some of the phone camera lenses. Zeiss are proud about their lens used in the upcoming Nokia pureview phone. Similar 5 elements 5 groups construction and all plastic. Plastic (lens elements) and temperature variations are not compatible.
I have now two lenses to play/tinker with to see if I can make them useful for my needs.
The Sigma 30/2.8 (announced price 189 Euros here) is an all glass lens. Sound more promising to me.