XF 2.1 Debugging: The upload failed because the file could not be written to the server Message

We are receiving complaints that our users can't upload Avatars with their profile. The error message reported is:

Oops! We ran into some problems. The upload failed because the file could not be written to the server. The site administrator will need to resolve this before any files could be uploaded.

We at first thought it was a space issue, so we did some clean up. But the issue came back.

What are the best steps to debug the issue? I didn't see a XenForo log option, so I turned on PHP error logs to try and capture more information. If there are other options, I'm open to suggestions.
 
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