Miele w2209i gushes from soap drawer housing

I removed the housing top cover and checked the myriad of hoses, none of which were blocked. One of these hoses, which branches from the top hose, attaches to some kind of chamber mounted to inside rear panel. I wonder if perhaps this chamber could be blocked. Apparently it is only an intermittent fault and I only observed it once, after I’d reassembled. It seemed as if the top hose simply could not cope with the volume of water. This was on a rinse cycle, and every time the housing started to fill up and overflow, filling would stop and then resume after the water level had gone down (some kind of pressure sensor?). I noticed that there was a breather hose from the top of the tub, and I blew down this. Wasn’t really sure if there was any resistance, but I tested it several times afterwards, it filled continuously and didn’t overflow. But since this is an intermittent fault, I’ve no idea if I’ve cured it. Any suggestions folks?

Derek