Got it. I now have access again to the configuration tab of the WebUI.
I'll leave this reply for the benefit of anyone else who searches for this.
You can't simply uninstall the tvh addon and reinstall it from within the XBMC interface (one of the things i tried first).
But you can SSH in (i used putty on my windows box) and make the changes. In my case there files named (numbered) 1,3 and 4.
1 was for admin, and in the file admin was already set to 1. I was puzzled. But to make sure, I made the change for files 3 and 4.
just use the command
nano #
where # is the file number.
also, the path from root is:
.xbmc/userdata/addon_data/service.multimedia.tvheadend/accesscontrol/
and in that directory i found the files 1,3 and 4.
Thanks.