I think its a security measure to prevent the use of multiple ids on this forum.
You can test it as follows:
If you are already logged into this forum and open a second page without closing the first page down it will open showing you as logged in. ie you now have two pages open each logged into the forum.
If you log out from one of the pages and then log in again you will now find that the avatar box has disappeared and the community member tickbox is unticked and cant be reset.
I logged in as myself ,on both pages, to test it and found that it unticked the box and this removed the avatar upload too. So it seems that this problem occurs when you are already logged on and attempting to log on again from the same computer / ip address.