error: "upload folder is full." and post freeze even after attachment deleted

[EarthFurst]

Tried to upload a small (40k) jpg, but got error "The upload folder is full. Please try a smaller file and/or contact an administrator."

Related: After I use "Clear Attachment" link in hopes of posting, the "Post" button doesn't work.
(Work-around is to open new tab, and copy & paste into new tab and NOT attaching anything.)