I get this error frequently while testing. It usually occurs when I have an object(Word, Access, Connection, etc.) open and the application fails due to a programming error. This leaves the object "hanging around" in the operating system since the "close and set to nothing" code was not executed. It makes the object look like you're already using it and cannot have a second copy of it.
If you're having this problem on a new release of your PC/LAN/Server, check to see if the ODBC connections have been marked "reusable". I've only run into this once and don't remember if they should be set to yes or no.
If the object in use was on my PC, I can clear the error by signing off and back on again. If the object in use was on the LAN/Server, I have to sign off and power down the PC to clear the error.
I try to leave any application objects(Word, Access, etc.) minimized but visible. If my program fails, the application can be manually closed without rebooting. Saves a lot of time.