try to copy them into a new blank project to be sure. if you have 4.23 installed, try also in that one. open the map in a editor (or hex-editor) - keep in mind how big the file is it may crash your editor (depending on your editor), search for “++UE4+Release-” and have a look what it sais after that pattern. I guess you already checked the logs under the Save\Logs folder if the editor complains for something?