So I started experimenting:
Workaround A
If I delete my xnview.ini, the program starts, but I don't want to comb through all the settings and recreate them all over again.
Workaround B
I've re-installed 1.12.1 and this version starts perfectly well with my xnview.ini
I've attached the xnview.ini so you can test things yourself.
System:
Windows11 26H2 64bit
XnViewMP 1.12.2 x64
It looks like the startup bug that was fixed with 1.12.1 is back with 1.12.2.