Hi,
My guess is the firewall at your host or CDN. It is very uncommon, but sometimes heuristics can pick up false positives and block immediately. The most common is when submitting forms – aka changing the settings. Modern firewalls have a tremendous amount of rules so false positives can happen from time to time.
I recommend checking the firewall options on your host. There are usually lists of blocks and you can whitelist that specific instance.