Hello Joerg,
yes, "Kontakt 60" from the manufacturer you mentioned is well known to interact with older PCB material. The contents of the spray get soaked, and lower the reistance of the pcb. And this contamination is permanent if you spray too much.
this makes "funny" results on e.g .older boards or rotary switches: Once the board is soaked, it can catch fire when voltage is applied. Older roratary switches with pcb base get completely damaged..
So extreme care should be taken just to apply the stuff on the metal contacts only.
"tuner 600" is more mildly, I would recommend this to for maintenance for older T&M gear.-- but that's just my 2cts. ;-)
For pot cleaning this recipe helped my a lot of times: Drill a very small hole into the pot housing (1 mm drill/ 0.05 inch drill). Apply one drop of "Kontakt60" via a syringe into the hole (To get the stuff from the spray can: spray small quantitiy into a small glass, and fill up into the syringe). Now turn the pot a dzen times left/right. Re-apply a second drop and repeat. Usually, pots I treated this way did perfect service for 2-4 years -- so I guess this is well worth the efforts.
hth, Andreas