Thanks for all the details you've included here. It's certainly a strange situation that only two users are seeing this warning on their PCs while being fine on others. That signals to me that their user accesses for QuickBooks are likely fine and suggests something is going on with their PCs, as you suspect. I'll do what I can to help.
From what you've described, it sounds like it's only happening with purchase orders, which makes things even stranger. All the same, I'd like you to check a few things. Since I'm not sure how you and the other users are sharing the company file, my suggestions follow different paths. While it's ideal to share a company file using networking and multi-user mode, I know there are some customers who share the file via a USB or other drives. Let's start there.
If your users aren't all networked in and the file is being shared using a drive, the most important thing is to make sure everyone is using the same company file when it's their turn. If not, you'll end up with duplicate company files that will have different information and potentially even different configurations. It could be that those users are using a different version of the company file than everyone else, which will cause problems. This could also occur if the users aren't picking the correct company file to work from even if everyone is using a network.
The next thing I'd like you to check are Windows access permissions on those computers. If the PC's system isn't configured properly for users to access the files, you may run into errors such as this. You can verify the setup by reviewing the steps in this article: Set up folder and Windows access permissions to share company files
Next, try one or both of the following processes: Clean install QuickBooks Desktop or Reinstall using a Super Clean install. While I appreciate that you've already done an uninstall and reinstall, these processes are used when other troubleshooting steps aren't bringing you to a resolution. In this instance, they're worth a shot if the agent or agents you spoke with haven't taken you through already.
Otherwise, it may be worth checking in with an IT person to take a deeper dive into the computer and it's interacting with QuickBooks Desktop to see what you can find.
I hope this helps. If you're stuck again, don't hesitate to give our phone support team another call. In case you need it, the number again is 1-877-772-9158. Pro and Premier agents can help 24/7 while Enterprise agents are available Monday to Friday between 9 a.m. and 8 p.m. EST.
All the best to you!