I’m having the same issue. I have tried quick register/full register, also tried different client devices. Even wiped and reinstalled FOG 1.5.3 on Ubuntu 18.04 LTS instead of CentOS 7. Still having the same issue.

I recorded the process I did when trying to register the host here:
https://drive.google.com/file/d/136YeecO8ysLPGmc2kF-btCifNj4Rqbzy/view?usp=sharing