Ok, so this is a bit exotic, but hopefully might help someone if you happen to stumble onto the issue with GrapheneOS, or if it happens on another version of Android.
If the usual “muh login dun wurk, Invalid ReCaptcha bad” fixes (so “clear cookies, allow popups, turn off adblockers”) don’t fix the login issue for you, try switching off the WebView JIT (that’s “just in time”) exploit protections for the RoonLabs family of websites.
Not being able to login is an infrequent bug that’s affected every platform, iOS included. There seems to be a GrapheneOS specific twist, hence my post.
That you’re having issues is kinda expected: the Roon apps tend to be a bit buggy, and Graphene tends to be more strict than most, and thus will catch bugs that other ROMs don’t, which is why it’d be nice if RoonLabs used it as a test platform.
If you’re OK with lowering security a bit, long tap on each of the RoonLabs apps, app info, scroll down, enable exploit protection compatibility mode, dissallow hardened memory allocator is worth trying.