Hi! I have just started having exactly the same problem in a very new UM2+ ( not more than 2 days of use/printing).
I have three UM printers: One "old" UM2+, another UMS3, and recently I bought another UM2+.
In the two older printers I never suffered this problem , but in the newset UM2+ the bowden comes out from the feeder after 15 or 20 minutes of normal printing.
I have checked already everything as you also did.
I am also very interested to find a solution as it is a very new machine and I don´t know wich should be the reason for this issue.
I am looking forward a "materialistic" solution as I do not believe that a machine can modify its behavour depending on our spiritual status as geert_2 suggests .