I also have DDs defined on each VLAN and could remove it in the past (didn't try with version you have) so that should not be an issue. I wonder if some device leftover exists... if this is on Linux, can you do following on backup server:
grep "evice access" -A1 /nsr/res/nsrdb/*/*
So, you are looking at the value of device access information and you wish to see if any is defined for DD VLAN path you have or wish to remove. As this line is usually long and therefore escaped to new line, I used -A1 to list line below the one where I found the string I'm grepping.