Hrm, sounds like imgur doesn't play nice with safari mobile.
Understand, nothing about the imgur plugin is actually in our control. The entire thing is 3rd party code to "translate" the imgur embedded gallery api into a format that can be inserted into xenforo's post structure. None of it is customizable at our end. Whether it works or not is up to whatever black-box conversion happens on the github server of the author.
As such, I don't know whether the problem lies with the xenforo plugin or the imgur api itself - and both are just as likely as the other. All we can do is keep ourselves on the latest version, and hope.