My friend has the retail package for Trinity. I have the retail original game Sins of a Solar Empire. I purchased Entrenchment and Diplomacy on Impulse. He runs XP, I run Vista 32bit. We also had this error.
I found a solution after 7 hours.
The solution was for him to zip all of his Diplomacy MESH files (3 files) and send them to me. I deleted all of my MESH files in Diplomacy and put his in.
We did the same thing for Entrenchment. Now it works.
Now for how I got there.
He purchased his copy more recently, so we believed originally that my copy was out of date. My copy wouldn't update anymore on Impulse, so I deleted everything from my computer and reinstalled it. I started with my SoSE disc, and then downloaded and installed Entrenchment and Diplomacy. Same problem.
I deleted it all again. First I uninstalled using the control panel. Then I deleted the C:\Program Files\Stardock Games\Sins of a Solar Empire folder. Then I deleted the C:\Users\Destroy\AppData\Local\Ironclad Games folder. Then I opened Impulse and Downloaded the entire client. That didn't work.
Then He deleted all of his files using the same method as just stated above, and installed everything using his Trinity disc, and then updating with Impulse. Same problem.
Then he deleted all of the files as above, downloaded the entire client from Impulse, and we tried that. Same problem.
I logged into his account to make sure that the difference wasn't between Trinity and the original SoSE. I uninstalled everything as above, and then using HIS account, downloaded the entire Trinity Client. Same problem.
We found that in this state we could play the original Sins of a Solar Empire, but still couldn't play Entrenchment or Diplomacy on either Ironclad Online, or through direct IP connections. He zipped his Entrenchment MESH files C:\Program Files\Stardock Games\Sins of a Solar Empire - Trinity\Entrenchment\Mesh and sent them to me. I deleted all of mine and put his in. I did the same for Diplomacy, and now we can play.