I feel ashamed.
I tried to force the time_t 64 transition in unstable because the system was in a state no longer usable: more than 700 kept back packages and were unable to install anything new so after manually forcing the transition it became unusable, with a lot of broken things. So I set up a stable installation and I wanted Virtualbox for some virtual machines I use. I saw the fasttrack wiki page, I saw the instructions were for bullseye, and I assumed changing bullseye with bookworm will work, but I do not know why bullseye survived in the sources.list even after changing it manually at least two times. I do not know if KDE's klipper was playing with me pasting the incorrect data or what, but bullseye survived... I also didn't notice the error message trying to install virtualbox was referencing bullseye...
Bullseye and bookworm are totally different words but maybe for no english native speakers they are easily mistaken.... I don't know.
Now my sources.list is with bookworm, and virtualbox is working just fine, like it was in unstable.