Hi Mark,
Experienced a very similar problem when making an update to a Rise Course and then trying to upload SCORM to SF.
One quirk our team discovered is that all embedded filenames of images and videos need to be less than 15 characters (inc. spaces) otherwise we get an error when trying to import the SCORM as a content object - This wasn't always the case so may have been a system update at SF end.
I more recently discovered this problem expands further in that if you replace an image or video in a pre-existing course, the SCORM will be rejected by SF if any pre-existing images or videos exceed the 15 character rule (despite them being uploaded historically with no issues).
I managed to fix the problem by renaming all embedded content using less than 15 characters and then re-embedding them into the course. If you can't find the original images you can always unzip the SCORM and obtain them from the scormcontent -> assets folder.
Hope this solves your problem!