Nokia 6.1 stuck in "Download mode"
Factory reset

For some reason, my device was on slot b:

$ fastboot getvar current-slot
current-slot: b
finished. total time: 0.001s


After changing to slot a,

$ fastboot set_active a
Setting current slot to 'a'...
OKAY [  0.009s]
finished. total time: 0.010s
$ fastboot continue 
resuming boot...
OKAY [  0.319s]
finished. total time: 0.319s


I got a menu offering a factory reset, after which the phone booted into the OS.