I also had trouble with D2D activation using the U-Boat Missions upgrade for my version 1.0 boxed version of SH4. Modding gets you into all kinds of trouble with the original game files and I regularly had to reinstall a fresh copy when things got screwy. I also kept a saved copy of the upgrade download when needed, but like you, the activation would fail. My solution was to simply call tech support at D2D, and a "real" person would take the call.....work his magic on his end.....and the next time I tried the activation it went through. I did this several times without a hitch.
When I learned D2D passed off their work to Gamefly I decided not to open any more can of worms. I looked for a CD version of the Gold Edition and found it on
iNetvideo for under $9.00. Received it within 3 days of purchase. I see the price has dropped since I purchased, down to under $8.00 now!
Still higher than buying SH5 for only $6.49 (tells you which game is more desired).