Your screen shot cuts off the right column of that screen where one would normally see 'Feedback' and a link to the file(s) submitted.
In setup of the assignment, what are the Feedback types?
Error suggest more than one document was submitted?
Does the path to Ghostscript check out? and has that been tested?
Cron job/task for conversions running OK?
There is also an addition ... path to pathtioppm - Poppler. Comment under that path setting says:
"Poppler is a PDF rendering library which includes the tool pdftoppm for converting PDF files to PNG. Performance is generally better than when using Ghostscript, particularly for large files. If available, pdftoppm will be used in preference to Ghostscript. On most Linux installs, the path can be left as /usr/bin/pdftoppm. Otherwise, you need to install the poppler-utils or poppler package, depending on your Linux distribution. On Windows it is provided by Cygwin installs."
The bug report you linked to was fixed and had a process to replicate the issue. And, your version of Moodle is behind ... highest is now Moodle 4.0.6+ (Build: 20230210)
In addition, have you checked server's error logs? Is site using a web application firewall (like mod_security/other), which might prevent conversions?
'SoS', Ken