Not sure if my earlier post will make it through, so:
Thanks Brynn for your help, you've helped me fix it!
I had to adjust the Boundaries like you said. It took me a while to find the right setting, though, since I had a lot of effects in that document, because of trial and error. Now it looks good.
Apparently the reason this happened was that I must've at some point selected all the objects and applied the effect to all at the same time.
As for the effect itself, before fixing it, I took a look at the XML code and found out it's called "pixel smear". So, bonus points for me, I guess...
Again, thank you for your quick response time and your support.