I had same problems with DCIM and other media folders on N900. The solution I found was:
1) Backup any file in problematic folder. Use usb pc connection or other method.
2) Remove entire problematic folder. Use Filebox (file manager wont allow). Install it if you dont have it.
3) Take some photos and videos with defalt camera software (this will recreate the deleted folders automaticly). Notice that you must select internal memory to save files (not sdcard)

After that, all must work well.