Problems with attachment uploading after upgrade

[Insert Username]

I've got one user reporting that he's getting a limit error when he uploads an attachment. Others don't even have the option to attach when creating a new post or reply, but they do when editing a post. I'm stumped.

I've checked the settings. Their usergroup has permission to upload, and the limit is set to zero (which is unlimited).

Is there a setting somewhere that limits the total number of attachments per user? I can't find one.

I'm getting frustrated, because it all worked fine before upgrading to 3.6.1 and, without changing any settings, now it doesn't.

Any help would be greatly appreciated.