I have... I have had it to me back in the day with Windows 8 and then when Windows 10 first came out. It was all related to Defender and being on the manufacturer installed OEM Win 8 and updating to the free version of Windows Home x86 back when it first came out. Upgraded to x64 Pro and it never happened again. Not insuring this is the situation at all, since we have no OS, AV, etc., info to go on. For all we know they could be on a Mac or Xerox build. No info was given. Since this is just one odd potential possibility that some may not suggest, I would at least give it a try.
Running an AV scan won't hurt, unless it's the Antivirus causing the problem. Also, it depends on the level of security you set your Antivirus to and also what level user you are. If you are not the system/network admin yes, you can be locked out of the computer in front of you by the computer/network because the infrastructure doesn't recognize your account as the owner.
Again, maybe not a common issue, but not all that uncommon...
I did tech support work for about 3 years before I started my own computer maintenance/recovery business (but I got so overworked so fast I quit that shit -- after buying a new car and building an addition on my house). I gained local temp pseudo-fame as being the first person in my region to have manually figured out how to beat a cryptolocker ransomware when that was a thing before MB and EEK released security cleaner/fixers for that. Basically it was easy after playing with a few of them in sandboxes, they would typically create a reg entry with their own decryption key stored on your computer. Once you had the key you could run a bat that was installed in your temp files, enter the 256bit encryption key, and decrypt their personal files. This made me locally famous for a time and all the businesses were throwing their money at me. LOL!
The scam was you would call the number on your screen when you seen all your files were encrypted, then they would make you get a prepaid card for $500 then transfer it to a Bitcoin account, then download TeamViewer, and let them unlock your computer. GeekSquad and others were charging $300 to do it and their method often didn't work, sometimes it did nothing, and you still had to pay. It took me less than 1 day to figure out and only a few minutes per computer to fix. So I'd clean your network for $75 and then install a good AV Suite (BitDefender back then) with reliable antimalware and anti-PUP protection with pop-upblockers and web anonymizers for $150 more and then would do follow up maintenance and emergency calls for $75/hr. I was killing it... but, also myself as I had another full time job and I was doing like 90-120 hours a week.
Anyway... I kind of rambled and cannot even remember why I started talking about this or what my point was... Hopefully I addressed it... LOL! Getting old sucks balls... Alzheimers and those golden Kevorkian years, here I come...