I've a problem with Search Console. For some months now the "Coverage" report has been detecting many "5xx" errors caused by URLs that don't exist and have never existed on my site.
What can it depend on?
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It looks like you have two issues:
First, those inexistent URL's are being indexed in the first place, that probably menas that you have reference to them somewhere in your HTML. You can use an SEO tool like ScreamingFrog to spot these broken links.
The Index Coverage report shows the Google indexing status of all URLs that Google knows about in your property. source
Second, Your website is not handling correctly the 404 pages; Make sure that any inexistent URL is responding with a 404 status code so Google effectively know this pages no longer exist.
What you need to do?