Mark Richard Alexander Berginski

I, too, have the same issue on my server.
I tried to add someone to the Whitelist → 403
I tried joining with the account and then use the command again → 403 again
For some reason though, it worked, when I tried again a few minutes later:

I am running Paper version 1.21.4-221-main@c467df9

I have the following DNS Settings:

It was the first time in over a week, that I used the /whitelist command, so I highly doubt I am over some rate limit.