I followed what you said but still have the same error as before.
Here's what i did:
1) open 3ds MHXX save (System)
2) find my name (starts at offset 6746)
3) copy the next 11D088 blocks (starting from 6746)
4) open Switch MHUG save (System)
5) start at offset 6746, overwrite the next 11D088 blocks of data
6) save and restore using Checkpoint.
Did I miss something?
So instead of step 5, I search for the first instance of user_name in the MHGU file and replace the next 11D088 blocks of data. The reason why I want to try that is b/c at offset 6746 it's all 00s in the MHGU save.
I was able to get slightly farther before it craps out with the same error (actually gets to the MHGU screen).