This will almost definitely be a temporary issue with the server hosting the Mastodon instance rather than an issue with the app. All that you can do is wait for it to resolve itself or be resolved by the server maintainer.
If you go to the server with a web browser, sometimes whoever's running it will have announcements about what's happening and when it might be fixed. Not always, but it's worth a try.
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Server issue
This will almost definitely be a temporary issue with the server hosting the Mastodon instance rather than an issue with the app. All that you can do is wait for it to resolve itself or be resolved by the server maintainer.
If you go to the server with…
If you go to the server with a web browser, sometimes whoever's running it will have announcements about what's happening and when it might be fixed. Not always, but it's worth a try.