Reparing the plastic isolation on String ladders  

Remove the old coating and have Rilsan® applied at the applicable service provider.

Thanks, but removing the coat completly isn't really an option ... besides probably beeing expensive. Otherweise I could buy them new from String, as they reissued the shelfing system. At least some variants as they do not offer that much measures than in the past.
Any idea for just repairing the damaged spots?

You could try white sugru
https://sugru.com

Thanks, I'll give it a try! Just ordered a pack of snow whites ... shipping to Germany is quite fair!
I'll also have to clear all the ladders, as some are slightly yellowed. Some more, others less. Any ideas for a thoroughly but gently cleaner?

Sugru is great.
Prodip could be another solution for repairing larger surface but it's less sturdy than Sugru.
http://www.prodip.eu/

Thank you as well! Looking forward to try out the Sugru!
Any ideas on my cleaning matters?

We use this product with good results: HG plastic garden furniture restorer.
http://oc.hg.eu/products/show/id/678/hg_plastic_garden_furniture_restorer

Great, their products are available in Germany as well. Thank you very much, I'll give it a try!