Solution
Removed a RAM stick. Then the system was bootable. Weird state gone.
Ubuntu and Windows worked fine. And reinserting the RAM stick didn't produce a relapse ;-)
Preventive measures
Changed grub menu order, placing Windows at top (default). Having Ubuntu as second option shouldn't be a problem, because I mainly use Ubuntu when working (= focused mode).
The simple way to do it would be to edit /etc/default/grub and set GRUB_DEFAULT=2 (since Windows was the third option in the grub menu).
But that wouldn't have changed the order, it would only have changed the default preselected option.
And then I would have to hit the arrow up key twice to get to Ubuntu – in essence equally unsafe as the starting point (only for another OS).
Instead I changed the actual order, listening to advice at Editing Grub2 menu entries manually in Linux Mint (3rd comment, by bces0605), and consulting the grub2 manual: Change script boot order by changing numbers.
So here we go:
# backup existing grub.cfg
sudo cp /boot/grub/grub.cfg /boot/grub/grub.cfg.bu0.before-move-windows
# list all entries (even unused)
ls -la /etc/grub.d
# move windows entry to top
sudo mv /etc/grub.d/30_os-prober /etc/grub.d/09_os-prober
# notify grub
sudo update-grub
Then I diff'ed the /boot/grub/grub.cfg changes (using Meld) – see grub.cfg diff – to make sure that things had worked as expected.
They had – nothing but the order had changed in /grub.cfg.
And new grub menu order is:
- Windows bootloader
- Ubuntu
- Advanced options for Ubuntu
...which my fumbling fingers supposedly should be able to handle safely.
Beware that the Windows entry in your system may have another name than 30_os-prober.
