Compressed (zipped) folder is invalid and won't load on LMS

I have recently made changes to an existing and currently published captivate project on our LMS (saba). The published version was in Captivate 9.

We have moved to Captivate 19 and I have made minor changes. Key change has been the attachment of 2 PDFs . The projct previews without a problem. 

When I publish it and send it to my colleague to upload on the LMS the upload is unsuccessful. the latest message is:

Windows cannot open the folder. The compressed (zipped)folder ...........is invalid.

I hadn't changed the quiz settings.  I am running out of options. not sure if  its the new version that has caused the problem. I have been finding my existing projects are not as stable sincewe switched to 2019.

Any help would be greatly appreciated.