when adding a supply ship, the standard "liquid cargo carrier" to the master mission of an external campaign, when pressing "generate" in IL2 it locks up, with the following message:

 

why does it do this? this is the only thing different to the file that works ok?

That doesn't make sense.  The tanker is included and the error has nothing to do with it.  Can you cut and paste out of the master mission file the [Chiefs] and [NStationary] sections?

I could, but the NStationary text length is longer than this forum allows in posts.

 

Australia.txt

In your mission file:

  0_Static ships.Ship$Tanker0 1 90546.85 52924.38 600.00 0.0 0 2 1.0

 

Non-modded mission file:

  0_Static ships.Ship$Tanker 1 246629.27 38619.96 360.00 0.0 0 2 1.0

 

You can see that your file has renamed the Tanker to Tanker0.  You'll have to add it to the appropriate DCG files (gobjects.dcg and gunits.dcg).