One thing to check is the file type extension coming from your camera. Different cameras will use different file formats and the general ones like JPG/JPEG are always an acceptable type.
The iPhones for example, on some models, switched to an .HEIC format, maybe they still have that. If that was not a format compatible to the StoryMap system, then it would generate an error message like you described. If you needed a JPG, you would have to do a conversion over from HEIC. So, check to see that your format is a type acceptable to the system.
If you have uploaded the same file format before, then it is possible that an updated version of that format is now in use on your camera via some software upgrade and the new version is somehow a little different and produces the error message.
If you can't find something obvious pretty quick, you might try tech support.
Good luck.
