I have also had the 404 errors crop up suddenly. 2.4.3 worked fine, I upgraded to 2.4.4 and where GDE used to display a file, now there was nothing. Blank screen, nothing.
So I dumped the source code for the page and where GDE was called I saw an error about "404 document not found, use force='1' to skip this check." So I went back to the page and added that parameter. Now GDE displays...but just with a big 404 error and it looks like it's trying to reload the website in that window
Here is a link to a page where the force parameter is used:
Here is a link to a page where the force parameter is NOT used:
In both cases, I have determined that the PDF being displayed exists at the URL specified. I can copy and paste it into a browser tab and it displays fine.
I hope this is helpful. By the way, I have NEVER been able to used the Enhanced Viewer option without getting the 404 errors.