[Minor spoiler alert]
The second time I ordered a slave from Sebastian (a Taurus for Melissas Quest, the first time it was a male :P), the slave's rank was "person", not "slave" or "poor" etc. I could not interact with the slave.
Changing ["origins":"person","originstrue":"person"] of the slave in the save file to ["origins":"slave","originstrue":"slave"] fixed the bug. I ordered 2 more times since then and could not repeat the bug.