I installed X3 while leaving X2 installed.  In retrospect I probably should not have run the Dimension Pro 1.5 installer again, but there were no instructions to the contrary.
 
Now I am completely hosed.  I put in the new serial number, of course, when running the installer.  When I try to use the synth, it pops up the registration box.  I enter the new registration code, and it I get a message that it is invalid.  I have triple checked it with my online account.  I am putting it in correctly. 
 
I even did a full uninstall on Dimension Pro and went through the install one more time.  Same problem.  Now I can't run Dimension Pro under wither X2 or X3.  Anybody else having this problem?
 
I'm really screwed until I get this fixed, because all of my current projects have that synth in them.  I did clone my whole system on Thursday, so I am about ready to scrap the whole X3 system and fall back to my working X2 system.

Make sure you are in Administrator mode before authorizing Dim Pro.  I got bit by the same thing but I eventually remembered solving the problem a few months back.

If you have a recent clone/image of the partition, then what's the problem?  Restore back, and work with X2 until you are ready to upgrade again.  I can understand the temptation to play with the new version, but honestly - you've only lost a bit of time - that's all. 
 
If it were me, I wouldn't risk corrupting the system further by troubleshooting - especially when you can just go back to a working state with X2.  Get back to making music, man!

Mod Bod
Make sure you are in Administrator mode before authorizing Dim Pro.  I got bit by the same thing but I eventually remembered solving the problem a few months back.

That did it.  I installed Dim Pro in Admin mode, but I wasn't in admin mode when I entered the code.
 
Thanks.
 
It would make a whole lot of sense for the registration window to mention that.

IF you still gave problems you may need to edit the serial in the registry. I found a post that explained the registry keys involved a few days ago using google to search the forum.