"Subscribe to this forum" 404 error

When I try to subscribe to a forum by clicking "Subscribe to this forum" i get the 404 error page.

I got, too.

Code:

Not Found

The requested URL was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

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The error depicts that weather the subscription page has been removed or the API or plugin integrated behind is not working.