If you select a ship or warship interdiction offline then when the map is displayed in order for you to select the target only the targets that have associated convoys are displayed.
However, in multiplayer there is no target selection display from the eaw.exe, and if you select a target which has no associated convoy there will be problems.

This is why we use the mission editor to make our selections, and in the latest version I have made the mission editor display the ship convoys with little white lines and a black circle.

Clicking in the black circle brings up a little box which shows the name of the target associated with the convoy:

In this case it is Martuba, which I selected as the mission target.
I then built the rest of the mission, saved it and flew a host-only flight at GameRanger

The parameters screen:


The briefing screen


Ready for take off



Here we go:



Checking the map:


It all worked well, so there is another mission in the Library