This is something that we already mentioned in the FAQS:
If you are using a web application firewall/proxy like StackPath, Barracuda, Cloudflare, Juniper, etc... try to temporary disable it to check if that's the cause. If so, you'll have to whitelist these endpoints: login/token.php, webservice/, lib/ajax/, and /tokenpluginfile.php

But we could just analyse the result of the login/token.php and grep for any mention of StackPath, Barracuda, Cloudflare, Juniper (I suppose there will be libraries doing this) and notify the user