Try this for the last time
Are you running an Anti-Malware/Anti-Virus App?
Yes I have McAfee and I accept Windows 11 security updates as and when.
Thanks Anke and Boban for your suggestions. I'm tied up next couple of days, but will try them when I get time.
Right - ensure that App Inventor is added to the 'trusted' or 'white list' of McAfee so that it does not intercept App Inventor and damage your project files. In my honest opinion, having used Windows since it began, Windows built-in security is all you need. Third party anti malware has a habit of producing false positives too.
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