Cannot add some instances

In a similar fashion with this issue, some instances cannot be added. This was not a problem with v1.7.0, but it occurred after upgrading to v1.15.5.

What happens: in the Federation section, when we try to add a new instance, sometimes it works and it is added, but sometimes it says "Error: Request failed with status code 404". This happens, for example, with LaPunta and AgendaAppenninica.

Here is a screenshot of the logs: https://img.tedomum.net/data/Screenshot%20from%202024-05-06%2010-50-03-b2e4f9.png AgendaAppenninica was not added, while LaSitua was.

AgendaAppenninica is up to v1.15.4 and has LaPunta as one of its friendly instances: so at least we know that LaPunta can be added.