The issue log is for bugs in Sigil, not questions. Supporting or investigating issues on very old and even recently old versions of software just isn't possible.
But since your colleague has the same issue on 0.5.3 my reply should help resolve the problem.
Note that the Cover semantic can be applied to both Images and to HTML Files. You may have applied the Cover semantic to your image - but you must also create an HTML File and insert the image into that file and mark the HTML file (preferable making it the first file in the book) as a cover to stop other HTML files being marked as the Cover.
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