First, I resized the Windows partition using the native Windows disk partition tool. Then, I've disabled the secure boot, so I can boot using the USB drive.
This happened probably because the /boot folder was encrypted, and the passphrase happens before grub loads. At this point, the additional keyboard layout is not set up, so I probably typed the wrong password.
After a few seconds (as the grub decryption is slower than the root decryption), I received the following error and was left in the grub rescue state. This behavior is described here.
As I'm using dual boot in the same storage device, I naively installed Manjaro entirely within a single encrypted partition by choosing the "Replace partition" option during the partition installation step.