I have not tried uploading other types of files; only avatars. So the issue, if any, may not be isolated to avatars alone.

Ah shoot, correction in my description post:

"However, when viewing forum posts or profiles on the secondary site (2nd.com), the avatar image is broken as it's trying to reference an image stored on 2ND.com"

Please confirm you have your board_prefix setting in _config.php correctly set. This is your forum base URL.

If you can confirm it is correctly set, I'll reopen this issue - but a code review tells me that it should work okay.

Chris,

I have set the board_prefix for both sites (even though I'm not sure if this needed to be done on the primary site too).

I was mistaken though, I misinformed you. Here's what's happening:

Upload avatar on 2nd site, file gets stored on 2nd site (should get stored on 1st site)
Profile view from 2nd site references 1st site
Profile view from 1st site references 1st site

It seems that both sites reference the correct URL, but if I upload through the 2nd site, it doesn't store itself on the 1st.

However, if I upload the avatar through the 1st site, it displays correctly on both sites.

To confirm, the correct value for board_prefix was already configured for the secondary site but not for the primary. So I went ahead and entered it for the primary, but the problem still persists.

Fixed sources/forum/cns.php attached.

Uploads can only be stored on the site they are uploaded to, as there is no data-saving connection between the sites. However, it is supposed to use absolute URLs appropriately to workaround that and there were a couple of bugs.