Okay, there were a few issues going on here. First, there was a minor bug/upgrade that needed to be made to the Variable URL Redirect web server plugin. I've updated the download here. I also updated the Installation instructions on that page. Note for Indigo 2021.1 the installation folder path has changed. The Indigo 2021.1 installer probably copied your old plugin to the correct location, so you should find the old version there. Replace it with the new version.
However, there was also a more general bug in Indigo 2021.1's new internal Web Server that caused that plugin to fail to load and start correctly. I have that fixed in 2021.1.1, but we haven't built an installer for it yet. If we end up building an internal beta installer before we release it I'll send it to you so you can try it out. Go ahead and install the new Variable URL Redirect plugin now though, then all you will need is the new installer once it is available and it should start working.