On the day of shooting, I came in early to check the setup.
The wireless lav mics were causing trouble, some of them did not seem to work while the others were producing horrible feedback and noise. Luckily I came prepared and had encountered similar problems before.
The person who rented the kit out before me must have mistaken the receiver lav with the transmitter lav and plugged the microphone into the wrong one. In addition to that, two wireless lavs were set to the same frequency, causing noise and problems.