It basically just automates searching the subreddit name and showing you similarly named communities. If it's not working, you could instead go to lemmyverse.net, set your home instance using the home icon, and then search for all your subreddits. Links you click will open in your home instance and you can just click subscribe.

No good for me, guess I had too many subs. In chrome I get “Header overflow” and in Safari I get “Error: URI Too Long”

It worked super well for me (a month or so ago). Had to find the multireddit link on the website, but then it all worked out well in here