Thank you all for all the great input!!
I will try the L-cysteine option. Regarding solutions, those are prepared daily from a larger stock, that is finished after about 2 months (all stored in a cabinet away from light). I ruled out the solutions though, as the same are used on other 5 instruments of the same kind, and those work fine. On those problematic systems, all parts that touches solutions have been changed and replaced by completly new ones.
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