Sorry Tiger. I've been supporting CFS1 since soon after it was first released back in 1998. I reckon to know as much about it as anyone here. I've never come across this problem before in over 5 years of troubleshooting all kinds of problems. I can only suggest an Uninstall/Install which fixes most problems with CFS1 without affecting your addons. You would have to reactivate any 3rd party scenery via the Scenery Library. If that doesn't fix it a "Full Uninstall" should do it. Uninstall from the option on the CD then exit setup & delete the CFS1 root directory folder before doing a completely fresh install. Back up any addons you wish to keep first.

Alternatively, you could try a duplicate install of CFS1. Rename the CFS1 root directory folder to any other name. Then insert the CD & install in the usual way. You will then have 2 separate installs of CFS1 that will run independently. This is useful for troubleshooting & the most effective way of backing up.