filemanager module breaks module administration page

In Drupal 4.7:

I move the File Manager module into the modules directory, click the 'enable' box on the module admin page, and click 'save configuration.'
At this point, my module administration page returns a blank page rather than "your configuration changes have been made" as expected. I AM able to see filemanager config options under settings admin, but when I install the "attachment" module, i can't enable it, because of errors in the module admin area.

If I move filemanager.module out of the module dir, this doesn't happen, and module configuration changes are able to made, no problem.

I'm not a programmer, so not able to track down the source of this bug, but wanted to let you know.