Thanks for the answers! Sorry for the late reply, been quite busy.
That could very well have been the issue. I'm currently not near the printers, or I would give it a go. What I can say is that we upgraded one of our two UM2 to a UM2+, and this seems to have fixed the issue completely. It prints perfectly again, leading me to think that the geared feeder fixed the issue.
When I'm in tomorrow I'll give your idea a try Geert.
And thanks again for the help!