Here are two possible workarounds:
1. Exit from the 80-column card firmware with ESC-CTRL-Q, then execute CONVERT.
2. Enable 40-column display with ESC-4, then execute CONVERT.
In both cases you restore the 40-column screen, and the program works correctly.
This situation occurs in the following configuration:
1. ProDOS Developer's Disk V 1.0 C
2. ProDOS system file dtd 01.11.83
3. Convert program version 1.1, dated 01.11.83
4. System configuration: Apple IIe and Extended 80-column card.