Can't assign access permission to media sources

I'm running Revolution 2.2.4, and I can't assign an access permission to any media sources. I don't know if this affects prior versions, since I've never set up media sources before. Here's what happens:
1. I right-click on a media source and select "update media source".
2. Click on Access Persmissions, then click on Add User Group.
3. The Add User Group popup appears, but no matter what I put in User Group and Minimum Role, nothing shows up in the Policy popup. Since the policy is required, I can go no further.

I have another installation of Revo 2.2.4 going, and the permissions work fine on that one. What could cause the popup to stop working?

Very good question... I've noticed some wonkiness with this too. Are you getting errors in your logs?

I would try this:
1. Clear cache and permissions
2. Re-run setup (this will test for various server issues)
3. clear your browser cache (sometimes the ajax requests that power those dropdowns get cached)