I had the same type of thing with my old 56K Serial Port modem. As I have a legacy free PC I had to connect it through a serial port USB adapter. I occasionally use to get an Error 797 port not open message and like you I had to remove the connection and then plug it back in. I never found the solution and after replacing the modem with a USB one it's never occurred again.
If you have a VIA USB controller it may be related to that. Updates are vailable click here but when I updated it didn't help.