Seems that some cargo missions are broken. I just got back from a "fetch cargo in Helios D2 , bring to Senega M5"..

Arriving with the 16 vismetal in senega M5, I get:
"Go Get the cargo from Helios System, Sector D-2, Helios Outpost(serco)"

And there I stand.

ingame time : GMT Sat Jul 34 18:49:49 2004

This happened to me a few minutes ago. I was in Bractus D-9 and had to go get 7 precious metals from Odia M-14. I went to Odia M-14, recieved the cargo, returned to Bractus Stronghold where it said, "Go get the cargo from Odia System, Sector M-14, Odia Stronghold (Corvus)." I tried to unload the cargo, but it kicked me off the server immediately. I then logged back on and I was in Bractus D-9 but about 2k from the station.

it also happened to me between helios and senega

Get Aqueous ore in Dau I-15 return it to Ithaca M-4 the rest follows danielky's experience.

and something else... Ordinarily if one logs off with an active mission, the mission does not persist. I tried to get out of this one by logging off and by having the station bugs kick me from the server. The mission persisted. I went back to Dau and was given the cargo again, Ithaca still didn't see it. I finally selected "could not compete mission". There was no standing hit., perhaps because I was using a low level character.

I have experienced this too doing missions for Dav L10 today, happened twice.

I wonder if the game isn't registering that you received the cargo from the first station... this is what you got when you bought cargo at instead of actually receiving them and bringing them to your destination (second station).