I tried some more things (like attacking her) but nothing changed.
So I went back to an old save prior to my interfering with the torturers and restarted from there.
Instead of convincing them to let me do the torturing, I took the last option : Tell them to let Minthara go.
This resulted in the beginning of a fight of course.
When the 2 cultists were dead, I told Minthara we would leave immediately. In the previous run I told her to wait so I could clear the prison from the remaining cultists first.
So we went to the door directly and this time she followed. I made her invisible and we rushed to camp without further fighting. She is there now, and the quest "decide Minthara's fate" is completed.
I'm not recruiting her yet, I want to go back with my current party and eliminate the cultists from the docks and prison , and free the gnomes and thieflings again. All this progress was lost by going back to that old save.
When Minthara thanked me for saving her, there was a difference between the first time (when she blocked) and this time. This last time, we also connected psionically and she saw all of our previous adventures and the prism and guardian that protected us. The first time this cutscene didn't occur. Maybe -- and this is just a hypothesis -- the previous attempt blocked because this cutscene didn't play and Minthara couldn't move until it was done. I base this theory on no evidence at all, just observing what was different.
For people who still are moving on in Act 1 or 2 and haven't confronted Min in the prison yet, these choices ( move her out as quickly as possible without any firther fights) may be an option that circumvents the bug.
