Normally it means that the threads that contain the referenced post has slid off the catalog because it's full, and that's just the way the chans work.
>>8601103 (From #11012)
>>8538300 (From #10931)
both still here, just loads really slowly, and I expect that for some reason it's blowing up the logic that creates a link.
Either way, not to worry, posts and threads are always being pushed off the main catalog. Archiving here is all about doing it as you go. There's no way to get everything all at once because much of it isn't here any more.