It can also happen with beggars and Torgall. It's indeed nonsensical, but also funny and harmless, so I'd leave it as is :)

I would also leave it as is for the same reason.

I'd just have it replace "cookie" with "wall". It totally changes the meaning of the 'fortune', but still works.

Maybe "Remember to record a new message for this voicemail greeting." :)

It needs to be made to make a little bit of in-universe sense... if not, it should at least make sense as a message in a game. For the latter, I can see an obvious one:

"Remember to come up with a new wall fortune message." (perhaps it could be in german, to improve the effect)

For the former, perhaps something like:

"Replace this message with one of your own when casting this messaging spell."

"This wall is intentionally left blank."

I just got a similar one:
'A mouth appears on the wall and says: "This cookie will make you pay a high price.".'

As a side note, shouldn't there only be one full stop in that sentence (inside the quotes)?

I don't know if this really counts as a bug. Beggars, Fortune cookies, and Mouths On Walls all seem to have the same set of messages to pick from. To make a seperate one for the walls, we may as well make seperate ones for beggars as well, and that's a bit much work for Thomas when he's already got a lot on his plate. Yes, I can reproduce this, but no, I don't think it counts as a bug.

Fixed.

The quickest solution is likely flagging some messages as cookie-only, so that beggars, Torgall, the Confucius book and mouths on walls will just skip those.

Anyway, I wonder how this was "fixed".

Marking messages would be an alternative solution to what ADOM currently does. Currently (as in p22) ADOM differentiates between random cookie generation for written contexts and for "other" contexts (fortune cookies being written, beggars being "other" e.g.). For "other" contexts it rerolls certain randomly selected messages. And in p22 the "reroll condition" is broken causing an endless loop.