I've had some strange problems when reassembling my UM2 with the "Olsson block" for interchangeable nozzles.
I find the brass block always ends up too low and touches the aluminium fan shroud. This block is pressed downwards by a spring.
There's also not much transverse room between the nozzle itself and the shroud hole. The interchangeable nozzle seems to stick out further than the standard one but maybe that's because it's interchangeable.
Anyway, when the brass block or nozzle touches the fan shroud I get heater errors. I tried sandwiching the white teflon ring above the fan shroud and around the nozzle, but the heat and pressure made it melt.
I don't really understand what's going on with the spring because it pushes all the brass components (Olsson block, heater, nozzle) down onto the fan shroud at the bottom. Further up, there's a steel ring around the coupler that looks like it could rest on an aluminium horizontal piece to relieve this strain, but it doesn't, there's a 2mm gap.
Edit: on this diagram
https://ultimakernasupport.zendesk.com/hc/en-us/articles/115004172206-Printhead-Diagram-Ultimaker-2-
...the "isolator nut" is flush with the metal plate below it. But for me, there's always a gap and the spring is pushing the brass components below onto the fan shroud instead.