Setting/finding Android 7 encryption password
It turns out that there either is a bug in TWRP in that it won't accept a blank password (or that there is a default password in Android 7).

In either case, if I set the password under 

Settings -&gt; Security -&gt; Screen lock -&gt; [PIN] -&gt; Password -&gt; Require password to start device


(which I have never set before), then this password can be used in TWRP and must then also be used to unlock the phone instead of the PIN.