I have been trying to resolve this issue for a week now this is my third email and I hope it is posted in the right place as advised by most recent response. I thought this would be easier to fix.
Vegas Pro 21 build 315 never had a problem before.
It has been a few days since I responded wrote/emailed regarding my issue that I said I think I have fixed it. I haven't had a response to that email. After further investigation and testing my findings are that any mp4 I create using Magix AVC/AAC mp4 I cannot use it again to add to another project as it is just not visible in Pro21 on my PC but if I use Pro15, or Pro 21 on my laptop, everything works normally. I reinstalled Pro 21 twice with no change. Any mp4 files that I created prior to 29th March all get accepted in PRO 21 on my PC but nothing since that crash date. There must be some fix. What do you think it is and if I can try something else. I can of course open the VEG file but it doesn't matter what I render it as it doesn't show in Pro21 on PC Perhaps build 315 needs to be an updated bug fix.