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Others are not broken, such as !twoxchromosomes@slrpnk.net.
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It's not a general functionality problem with subscribing or blocking. If you go to any thread in a community, the buttons work from there. It also works to sub from the community list.
I've found the same thing, if you change the sorting (Hot/Top/New/Old), switch between Posts/Comments or switch pages (even if the second page is empty), the buttons start working again too.
Alternatively, you can click the 'create a post' button and go back to the previous page; this works well for me.