I’ve been actively reporting this to Litespeed. They don’t seem to care much. Their "solution" (at least on Enhance OLS) is:
2) Edit the file /wp-content/plugins/litespeed-cache/src/rest.cls.php at line 150, and change // return true; to return true;.
Rolling back to 6.1 will allow you to request a domain key successfully.
Why does this happen.. well.. on Enhance the web server runs in a docker container, while not a proxy as such, it appears to the outside word like you are running behind a proxy.
I think Litespeed should not have "solved their issue" this way... but I'm not sure what is the best way either. Other OLS providers allow persistent configuration so a directive like this one: https://community.enhance.com/d/1105-please-implement-the-possibility-of-configuring-openlitespeed-free-version/15 on Enhance would also probably resolve this LS Cache issue.