Python – Iperf3 keeps hanging when running UDP test

I wrote a Python program that runs iperf3 in a for loop with different datagram sizes. I am using Python's subprocess.call function to run iperf3 like this:

 iperf3 -u -c 185.82.21.60 -b 100M -t 5 -l <datagram size>

My datagram size starts with 1000 and increases by 100 after each iteration and should run until it reaches a size of 2000. Every time I am running my program, iperf3 keeps hanging after the 6th time and starts using 100% of one of my CPUs.
After this I cannot use iperf3 anymore until I kill the running client process and restart the iper3 server.

Here is a picture to illustrate my problem: