For whatever reason the pulleys that come with Velleman VERTEX K8400 printer have very short thread that is easy to damage. Instead of replacing entire pulley and facing the same problem again, I came up with this simple solution. 3d printed ring with much longer thread is placed over the pulley. Mounting screw holds on the thread in the ring and passes through the original hole in the pulley, which has to be slightly widened.

This ring was designed for K8400 printer's pulleys, which are mounted on 8mm rods. Since this is extremely simple thing, you can easily create your own ring matching the dimensions of your pulley.

UPDATE Unfortunately this fix lasted just 2 months. Then the threads in the rings became loose. I printed rings with PLA and ABS may give the better result, however at this time I decided address the root of the problem by replacing rare 19T pulleys and GT2.5 belts used in VERTEX K8400 with much cheaper and widely used 20T pulleys with longer thread and GT2 belts. Waiting the parts to arrive. Will add another update here on whether the replacement was success.

UPDATE 2 I was able to successfully replace the pulleys and the belts. The details are here: http://www.thingiverse.com/thing:1857266