Hm, I got something wrong, I guess.
Tried the ..-gtk3-gui and was asked for admin rights. Does this work only with sudo rights? So do I need to have the user in sudoers?
EDIT:
Still missing something.
I change in the dir

./veracryp-..-setup-gtk3-gui

I get a (gtk) popup, which asks my pw,
I get an ok for uncompressing the installer
While the 2nd terminal just gives a "Press retun to exit"

That's all. No idea where I'm lost here..

Seems it was just a failed grub install. fstab, crypttab and mdadm looked unsuspicious, so I just re-installed grub. May be I had the partition wrong. Working now, but it is really slow loading the initramfs. But since it will be my NAS, that isn't a real problem (maybe I should have spend a bit more for the RAM, it's just 4GB).

Well, as usual I fail to get a working installation.
I end up with "Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)"

This is what I tried:
2 hdds ->md0 ->encrypted volume -> 2 logical volumes -> /data (ext4) and swap
1 ssd -> /boot (unencrypted, ext2) ; 2 partitions: encrypted -> vg -> lv -> both ext4 / and /home

Installation went through just fine (I skipped the random write on the disks since it would take ages for 16TB). Installed grub without UEFI enforced by default choice

So there are 3 ways, as far as I see it. (or at least for me.)
- there might be a bug fix for the fluxbox Version in bookworm/daedalus (bug fixing is still done after freeze, right?).
- compile v1.4 myself (might be a PITA)
- use another window manager

Maybe I got that wrong (or simply don't know enough about it).
My point is: the problem occurred after I updated from chimaera to chimaera and the question is, if that helps to find out, which package update causes the trouble, but I don't know if and how that is possible. 

What I can say: with 2 up to date versions of chimaera, amd64 has the problem, i386 has not.

And: installing gtk3-nocsd makes the problem worse (as in all windows have the problem then, not only FF/thunderbird)

Not sure how I would figure out if that is the problem.
I'll try that one on the live system. I'm a bit irritated, since it wasn't necessary before on chimaera.
EDIT:
1) glad I did that on the live system.. after installing gtk3-nocsd and using arandr every window has the problem, not just FF/Thunderbird
2) I can confirm to have the problem only with fluxbox, xfce has no problem (also using arandr there)
(sorry missed your edit of the previous post)

Thanks for that. Do I have to use the "load to RAM" option from the liveCD or will the install take place in RAM anyways?

/etc/X11/xorg.conf.d/ is empty, so it seems I haven't messed around there. Neither is that an option, as I use the NB on the road from time to time.

(your avatar brings back old memories btw)

It seems xrandr is the problem.
I set my monitors with arandr, from which I stored the xrandr command. (arandr generates a shellscript which contains the xrandr command). I stripped everything unnecessary (like turning off monitors, which aren't connected).

#!/bin/sh
xrandr --output eDP-1 --off  --output DP-3-1 --mode 1920x1080 --pos 0x0 --output DP-3-2 --mode 1920x1080 --pos 1920x0

Regardless if I use the shellscript or the command in a terminal, the windows of thunderbird and Firefox are missing afterwards.