I made just a few research back to early 2023 but I did not find anything on this lately.
And if it is not the right topic, please accept my apologies and forward this to the right place.
When hosting cryptpad locally (with docker in my case), using "localhost" or the local IP address, both inside the env_var and the config.js, is not sufficient for the WebUI to allow users to register. An error appears which is telling that an unxepected error occured.
When looking at the container logs (docker logs -f container_name), or looking at the process' stdout (tail -f /proc/PUID/fd/1), or even looking at the log files (cat ./data/logs) did not tell me anything.
But when looking at my browser logs, I could see that there was some issues with 2 requests to "/auth" with the referrer policy "strict-origin-when-cross-origin". It leads me to modify the httpUnsafeUrl inside config.js to : http://local_ip:port . The ":port" part was lacking to my understanding in the current doc.
It's now working. So maybe it was obvious for everyone and you should ignore this but I rather suggest to make this appear somewhere in the documentation.
Sincerly yours !