I bought my mini bow about a month and a half ago and after about 2 weeks of having it, I had the exact same problem. One morning I got up and saw that the filter was overflowing. Even after replacing the media cartridge with a brand new one, it continued to do the same thing.
I had a sponge baffle on mine (I thought that could've been causing it) but even after I took the baffle off it still overflowed. I've read somewhere else about other people having the same problem, so it isn't just you. I tried calling Aqueon repeatedly and could never get in touch with anyone. Your best bet is to just replace the filter with a better one. I hated the one that came with the tank anyway, it took up too much space and it blocked the light from the top.
You should try calling Aqueon and see if you have better luck than I did. I think they've released a faulty product and they should hear about it.