Yes, it's C4D on my Mac and I use the two Windows boxes to render. Many time, I disable the local Mac since it could slow down certain jobs. It is random though, so difficult to replicate. I just realized that the clients are not running Hotfix 1. I'm guessing it's always best to keep the versions the same. Should I update now, or do you want to test anything first?
Cinema 4D r20.059
2013 Mac Pro Trashcan, OS Catalina 10.15.2, Full-speed non-debug version, Build timestamp: Oct 8 2020 08:44:35, Version: 6 hotfix 1
Renderbox1 - Windows 10 Pro, v1607, OS Build 14393.2189, CPU: Xeon E5-2673 v4 @ 2.3 GHz x 2, 128 GB RAM
Renderbox2 - Windows 10, Pro, v1909, OS Build 1836.1198, CPU: i9-7900X @ 3.30 GHz, 72 GB RAM
Just an extra bit of info on one of the PCs from the Team Render Machines ping: "Connection speed test indicated a high data transfer rate. Upload-118.03 MiB/s, Download-112.17 MiB/s. Network speed shouldn't be an issue.