And now this message
2 mission(s) have been removed having been completed by someone else!
That is a normal message, missions are public you are competing with the others on completing them. It's basically the first to complete the mission is the one who gets the reward and all the others get this message and if they have a del process on the same file it will say "Target file not found!" because someone else got there first.
The gateway file delete should work fine now, just tested it.
Thank you