I just checked with the files that are provided with the Freepsace Port.
The mission objective that is required for the campaign to progress is for the Ratna to have warped out, aka "Ratna Successfully Escaped".
Gemini is supposed to dock with Ratna, tow it for a while, undock, then dock again, tow again and then undock again. After the second undock the Ratna should get its engines back and warp out on its own. Gemini is supposed to warp out after it.
Considering Gemini did not warp out the Ratna probalbly was not listed a departeted, which caused the relevant mission objective to not complete.
If you are playing the original there maybe an issue with the docking orders, as I see it there are a number of events referrencing docking proccedures for multiple ships but "on paper" I cannot say if they are properly sequenced. Retail FS1 and FS2 did not support a ship being docked with the multiple objects at the same time.
ps. Would you like to hear the word of our lord and savior, Silent Threat: Reborn