On new ISR routers with XE-SDWAN version 16.10.3+, you can only log in with the default password once. Over console, there is a message printed that warns you to change the password, but it can easily get lost in all the other console output.
Breaking boot and setting the register to 0x4124 doesn’t work as you would expect. To get back in (note, your config will be gone), break boot and config the register to 0xA102 or 0x8000.
rommon 1 > confreg 0x8000
rommon 2 > i
Then, change the configuration register back to 0x2102 and perform a sdwan software reset. This wipes out all the configuration that exists.
Router# request platform software sdwan software reset
After that, the device will reboot, login with the default admin/admin and this time—set a password.