Thank you very much for your both answers! I will run some tests and come back if it persists - but now that I know the right name of it and its possible causes, I feel confident. Cheers!
Image link: https://ibb.co/dxBEva (sorry, I could not embed it for some reason) I have tried decreasing speed, layer height, levelling the bed... and every now and then this problem comes back in very simple prints that should have two straight faces close to each other. This one was printed with an Ultimaker 2 Extended, PLA White, and standard configurations from Cura ("Normal" quality) - to rule out any misconfiguration of it from my part... Any idea? Thanks!