Alright, on zigzagjoe's request in IRC I decided to figure out what was wrong with the new NATfix, and have fixed it.
You can get the new version from here:
http://www.outpostuniverse.net/files/natfix21.zip(If you are using the old 1.x version of NATfix, please remove it before installing 2.1. You'll have to reapply it after installing 2.1. Unzip ALL files in the zip to your OP2 folder, overwrite if prompted.)
For the people who don't know what NATfix is, it allows you to host OP2 from behind a router, without getting the replication list error.
The reason it's necessary is because OP2 improperly detects the IP address of the host computer. It gets the internal network IP rather than the external Internet IP address. This is sent in data packets to the other players in the game, so when they try to send back to this IP, the data never gets there (because those IP's are only valid inside the host's LAN.) This causes the replication error.
Anyway, the fix recently released in the OPU4 update had a bug which made the hosting OP2 crash when the game was started.
The recent NATfix applies the patch in memory on game startup, rather than hardcoding it into the EXE file as the old version does.
This bug was caused by an invalid code patch (it would call an invalid memory address which caused the game to crash). This has been corrected and the new NATfix should now work.
The old NATfix will still continue to work, however we recommend you update to the 2.1 NATfix to prevent any possible problems. (The old NATfix is removed if you ever update the EXE. Also, your IP address is hardcoded into the file itself. Poor coding practice!
)
The new NATfix stores its information inside the Outpost2.ini file instead, so updates from now on won't overwrite the NATfix information. Also, this way, if you have a problem the IP address is easily changed or disabled by editing the INI file.
This new NATfix will be included in the next major OP2 update.
--op2hacker