If you see these, go to the record for this media object.
You will see the filename.
Check that the file exists.
If the media record contains "certificates/xxx.jpeg", then you need to check that the file data/media/certificates/xxx.jpeg exists.
If you created your GEDCOM file on a PC, then upper and lower case are the same.
On most servers, they are different. So it is common to find that a file is called PHOTO.JPEG, but the GEDCOM contains photo.jpeg
Harald, do you keep all media files in the data/media folder, or do you perhaps keep some media files in a subfolder data/media/subfolder/photo.jpg?
I suspect the "404 files" might be located in subfolders