It is most likely due to busybox being modified or no longer in /system/bin or /system/xbin. Without access to Root Explorer, it's unclear if busybox and su are still in one of those folders. What you'll need to do is get the latest version of Superuser and you will need to go through the Settings, on that app, and enable Root. Once you do that, you can then open up Root Explorer and check on su and busybox. You can also go ahead and get fresh copies of su and busybox and delete the old copies that were in /system/bin or /system/xbin (since I don't know if they were modified at all) and place the fresh copies of su and busybox in there. It doesn't appear to matter if it is placed in either folder. On my D2, it's placed in the /system/xbin.
**EDIT: Forgot to say, if you have any Superuser.apk (app) then get rid of them. For some reason I had more than 2 (older version and latest version) Superuser apps. I found 3 previous versions which seemed to have been causing issues to allow the latest version to install. So get rid of all of them, get the newest version from the Market.