I was going to suggest to: first try to tighten the screw/nut up a little and then try to loosen it. Sometimes this tightening action breaks the locked thread.
If that does not work, try to loosen it when the machine is
As Fresh said if it is under warranty take it back. If you damage it then you could void your warranty.
I had been signing some posts as JD. Sorry, I hope I did not confuse forum members. I will change how I sign off my posts.
John (is there anybody who signs of as John????)