"Go to depot" don't work on ships which have a line.
How to reprodece:
1. Load attached savegame
2. Open shipyard window
3. Apply line(01) to ship
4. Start ship
5. Open convoi window and click "Go to depot" button
Result:
Ship go to next scheduled harbor.
Note:
This will be solved after save and reloading it.
This is a generic schedule and points to some deeper stuff with virtual functions. HMM.
Should be fixed.