Hey @capetom,
I haven’t seen any reports of this coming in from anybody else.
Maybe try @Heroic_Nonsense’s suggestion in this post Permission denied. Couldn't start the internal web server on 127.0.01:21 - #2 by Heroic_Nonsense and see if it fixes your issue.
Also make sure the “Web Server Port” field is empty on that same page. Let RapidWeaver auto-select the port for you.
Let us know how you get on.