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Do you have the issue only with Excel files?
It sounds to me to Ransomware malware which encrypt files
I will suggest to call McAfee and open a case and do not try to use other antivirus because you could corrupt the files even more
Thanks, I have opened a case and will post feedback here.
Moved to Malware Discussion > Corporate User Assistance for better handling.
There are numerous mentions of it on the web, some good, some bad, and one which probably is good advice is to first try System Restore to before it started (this can be done is Safe Mode if necessary). If successful then turn off System Restore temporarily to erase the infected restore point.
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