nothing to do with a magna clean, looks like water ingress from the flue contaminating the insulation panel, is it a Vaillant ? have seen similar on a Viessmann
easily enough to fix , but you need to find the cause first, doubt if it is the condensate but anything is possible, I would be checking the flue first
You get this in an oil boiler sometimes I think it’s sulphur which is in all fuels
that xyz stuff mixed well gets rid of it never seen it in a gas boiler though. Bob
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