Ah, that is one of those bugged communities.
We also have one of those over here: https://slrpnk.net/c/buyitforlife@slrpnk.net
This happens if someone with an account from another instance is made moderator and then changes the logo or description of the community. As a result the Lemmy home instance of this community thinks it is a remote community even though it is still on the same server.
I think this bug has been fixed in the development version of Lemmy and those communities should become easily accessible again with the next version.
Edit: but it also looks like they moved to https://wayfarershaven.eu/c/fountainpens which is accessible from here.