There could be multiple reasons so we will need additional information like:
What product you use - VSE, ENS?
What are the versions of products and their modules along with scan engine/dat release version?
Does it happen for both scheduled and manually started ones?
Is it standalone installation or you use ePO management?
Any additional scan task running at the same moment?
Aside all questions above, one of the common issues is corrupted system files of the McAfee product and usually it is best to run latest removal tool followed by reinstall after a reboot.
I missed the fact it is under different section of the Community Portal so your instructions seem more relevant in this case.
Check if you have a drive which has been encrypted with Windows Bitlocker Encryption. If so, unlock the drive so McAfee can access to the data. This happen only with full scan, not with quick scan. It resolved my problem.