I’m glad it’s not just me then 
I’ll leave it running for a day and check back tomorrow. Thanks again !
A couple of days in and so far so good !
Sensor is updating @ midnight without having to restart HASS.
Thanks @petro for the solution ! 
Still puzzled why it didn’t work straight away the first few days .
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The error message is complaining about this template because it is attempting to subtract a string and a datetime object (which is an invalid operation).
{{ (due - day).days }}
That’s because this template produced a string instead of a datetime object.
{% set due = strptime(dstring, '%Y-%m-%dT%H:%M:%S') %}
It produced a string probably because strptime’s pattern %Y-%m-%dT%H:%M:%S failed to convert the value of sensor.omrin_papier from string to datetime object.
Post an example of the value of sensor.omrin_papier so we can see its format.