Varios 6s volute prevents pump from working

I have a varios 6s skimmer pump that's having a strange issue. It wasn't working - the red lights would come on on the controller every time. I cleaned it really well with no luck. But what I found is that it works perfectly fine without the cover/volute over the impeller. Once you screw on the cover, it stops working. In fact, you can put the cover on and start to tighten it, it only stops working once the cover is almost fully screwed on. It's like it's too tight. I have to push quite hard to screw it on, not sure if that's normal. And the pump is clean almost new - no obstruction.

Any ideas how I can fix this or what the problem is? Unfortunately the pump is a few years old so I doubt it has a warranty, but it's barely been used in that timeframe, maybe only a couple months max.