See images. I think this is the second time this has happened on this playthrough. The rest of the time, I get one or maybe two slave coffins, as described in this already-solved-in-1.9B2 bug. I don't know if the two bugs are related.

message on docking.png
message on docking.png
undamaged slave quarters.png
undamaged slave quarters.png
sav
Cimanyd-6176.sav
log
Debug.log
ferdinand 8 Aug 2020:

The small number of coffins is solved in the (not yet released) Beta 2. Still broken in Beta 1
However the other bug has not been reported yet. Surely have not had it in any of my playtroughs.
Perhaps you can also attach your savegame file and debuglog to your post?

cimanyd 8 Aug 2020:

Flew to a different slave camp with the same message (must be the other one I'm remembering) and attached files.

george moromisato 29 Aug 2020:

I can confirm this is the same bug. If (due to the bug) there are 0 coffins, then you get the angry response.

Although this won't happen again in 1.9 Beta 2, it will not retroactively fix this save file.