UPVC chemical resistance is ok but some mild attack
www.spilltech.com/wcsstore/SpillTechUSCatalogAssetStore/Attachment/documents/ccg/CBOOM.pdfIf the joints are done with a general commercial grade (Crap) adhesive with low PVC content it could be something leaching out of that.
If you have any rubber seals this is a high probability of being the issue as at best the rubber used is generally neoprene or lower natural rubber both of which are going to give grief as NBR (Nitrile) or viton should be used.