Quicktime error when combining 2 videos

Have 2 one-minute videos that I have recorded using my iphone. Am trying to combine it to a single video using the option File-Export As , but am getting the error "Operation Stopped The operation is not supported for this media."

This was working perfectly till a week ago. The help search has not been successful. To reiterate these are my own videos so there is no issue of copyright issues.

TIA