[Ultimaker 2 - is this a bad belt?]

geert_2, I think you have the answer. This wasn't happening until I installed a new bowden tube that I purchased from Amazon and it wasn't an official ultimaker bowden.

[Ultimaker 2 - is this a bad belt?]

Here's the left rear after several layers has been laid down. It seems just the raft, leftmost slot, and single hole is affected. Quite puzzling on the cause.

[Ultimaker 2 - is this a bad belt?]

This printer is approx 2 years old and this problem has cropped up while trying to print something that basically fills the entire build platform. The print is 7.5"(19.05cm) X 6"(15.24cm).  The pictures are of the brim being printed along with some holes. The bad area starts at about middle of the build plate and going left while looking at the printer. The left back is the worse with the left front not as bad.  I should note that I don't hear any type of slipping or chattering. The head movement is smooth throughout.

Left back

Right Back