Someone else posted something like this in the past...not sure if it applies to you or not:
https://www-secure.symantec.com/connect/forums/invalid-dn-syntax
Also, I would verify that what you are typing is exactly what is in Active Directory. See my example below:
FQDN.COM DOMAIN
--AD (OU)
---------LOCATION1 (OU)
------------------------Servers (OU)
------------------------Users (OU)
------------------------Global Groups (OU)
------------------------Computers (OU) <----location of object
---------LOCATION2 (OU)
------------------------Computers (OU)
In the above example the domain you would type in would be: FQDN.COM/AD/LOCATION1/Computers
I want to say, if the computer account already exists in the domain, you may need to just put in "FQDN.COM" (in this example) or it may complain similarly because the object already exists.
Hope this helps.