I have been trying to open a file, but keep getting the error:

"Filename" is currently in use and could not be opened. If the file is shared, you can use the Open Remote command to open the file on the network.

No one else is using the document, I triple checked. The Open Remote command didn't work. Anyone know how to fix this?

This is bad: it means you're trying to open a file that lives on a shared folder (shared by the OS) and somebody else already has that file open.

What you're seeing is the #1 reason for corrupted files.

If you want to share files, they should not be in a folder that is shared by the OS. The files need to be open in FileMaker Server or FM Pro. Other users need to use the "Open Remote" to locate the host and open the hosted file that way.