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That is very weird. In this case, I would still use thermal paste, as it is described in this manual:

Just use any plain thermal paste you got lying around on the interfaces between aluminium tube / cold end block and cold end block / cooling fins. Should help a lot.

When it comes to clogging, it's less a matter whether you use 0.5 mm or 1.5 mm retraction. Just don't use a retraction in the range of 3 mm and more. That will produce problems. The less retraction you can use without seeing strings / blobbing, the better (and faster).