It's not that big, it should process just fine in my opinion. Unless I underestimate the time it takes to re-encode the GIF into all the required sizes.
I think that has to be a setting in Imagick that I don't have access to. In fact, I have access to zero of its settings. All I have is an on/off switch. And the more I search, the more I find other forums reporting similar issues. It would seem that xenForo's implementation (and re-encoding) of animated GIF avatars aren't that great. And not that stable.
Sorry, I can't do anything else. Perhaps you can upload it here in this thread, I'll try it to see I can get it to work on my end.