This was an upgrade from XF2 to XF2.10 yes ? I just mean you've not just done a XF1 to XF2 upgrade ?
What actually happens when they go to account/account-details ? or maybe better to /account/avatar ?
Do they see the upload box/field/button area ?
Can you test on the default theme too temporarily if you are using a third party one just to be sure it is not that causing it, that seems quite unlikely but for 30 seconds to test that, it seems worthwhile to quickly check.
Is it all users ? Can you change your avatar ?
If it is only some, are they all using the same browser perhaps ?