Exporting creates M4V file and separate audio file - both cannot be opened

Ok, start off by saying I am new so apologise if this has been answered already. I have searched for similar questions butcouldn't find the answer. 

Am using a Windows PC (if thats relevant?) 

When exporting am using settings H.264, export video and audio selected, youtube 1080 selected. 

Exports now sometimes hang or when they do complete, places an m4v file and separate audio file in the chosen location - both files cannot be opened. 

Did try to upload the m4v to Youtube but had no sound and it cut off the last 4 mins of the video. 

Have uploaded 5/6 videos without issue before but since yesterday (07/06/21) all these issues are cropping up. 

Thank you for your time