Finally fixed it! I had the same problem with Desire HD. I tried to uninstall a music app I thought caused the problem and restarted the phone, no luck, still not working!! I tried accessing ringtones via personalize > ringtones and got the same error message. Third time lucky, I went into personalize > sound set and selected HTC Default 2010 and hit apply. Went back to ringtones and all is fine... hope this works for you!