Digging through this frickin' forum is very difficult, so here are some popular ones:

The invalid character that is stopping the transfer is the @ sign in the email address.  Remove that and it will transfer. (Personally I doubt this one)

Client is locked - Back up the client - Unlock and try the proforma again - Lock

From abctax55's theft from another forum ""..it turned out to be an invalid item within the charitable contributions. Apparently i's have an issue. It did  on both of these. Retyping the organization worked."  

Use Ctrl + W to very carefully look at the detail for "wonky" characters.