I have a screenshot of the crash dialogue if needed, which hints at an error in vectors.noe line 23 [though the problem description is a bit cut off], though this should be easy enough to reproduce.

Note that it only seems to occur if the game is started up in ASCII mode; if the player switches from (initial) tiles to ASCII at any point during the game, the bug does not occur.

Fixed by Zeno.

It still crashes on windows 8 prerelease 19 when attempting to open the old barbarian's monster memory while in ASCII mode with the following error message in the NotEye console:

handling NotEye error #8: error: did not yield [./common/vectors.noe:27: attempt to perform arithmetic on field 'x' (a nil value)

I couldn't reproduce this with Pre20 (tested on Windows 8 and OSX and starting directly in ASCII mode). Does it crash on all monster descriptions or just the old barbarian?

I just tested it, it only happens if you start in ASCII mode. If you start in ASCII and change to tile and then back to ASCII it does not happen. I have only noticed it with the old barbarian. I will check other monsters too.

Yes, I started in ASCII mode too. Zeno couldn't reproduce it either, so I think it is (finally?) fixed in Prerelease 20. :)

This still happens for me in prerelease 20 Win8. It happens when I start up in ASCII and with any Monster Memory. Only when I change to tile mode and back to ASCII does it not happen. Is there maybe some file I was supposed to delete between releases?

When the version with Sage features came out I had to do a full hard drive search and delete everything that has "adom" in its filename. Only then they (Sage features) started to work.
This why I almost exclusively play on server :D

Do you get the same error message as before or does it differ in Prerelease 20?

It looks very similar, but of course in prerelease 20 I don't have the console to read the full message, but this is what I can see:
"error: did not yield [./common/vectors.noe:27: attempt to perform arithmetic on"