I could bypass this by wiping /dev/sda1 clean and reinstalling windows boot files on it. I then had to use superGrub2Disk in order to access Manjaro.
However, it seems that now I have a problem when switching to manjaro after Windows:
Manjaro gets stuck on the textual loading screen. This seems to be fixed when rebooting the PC, but the problem occurs every time I switch after Windows.
After looking up the internet, it turns out that the problem is the IDE/RAID/AHCI config in the BIOS setup. But I cannot change it so I may be in some kind of trouble here if launching Windows prevents Manjaro from booting properly on next time…
Any clue?