I didn’t charge it “by force” so far.
It’s the last option before I’ll trash the battery.
I ordered now a new battery and a new USB-Port module.
I’m an electrical engineer and have the appropriate equipment to safely charge a Li-* battery.
Update
I left the phone charging with a notebook charger for ~48h.
After that it booted with 12% battery. I was not able to charge it further during operation.
Now it’s turned off but still plugged in. No sign of further loading.
Conclusion
First the phone was rebooting over and over with 1% battery.
48h charging to 12%.
No sign of further charging.
Neither while running (bolt indicator)
Nor while off (Led, vibration)
I guess the connection between the charger and the phone must be pretty high impedance.
So that almost no current is flowing.
This could explain:
long charging time
cold charger/phone
no loading indication (maybe several mA are needed to indicate charging
After the first successful loading period to 12%, I managed to port my 2-factor authentication, etc. to my notebook, so I’m fine right now.
The phone was turned off at 7% battery.
After two hours of charging (turned off), it has 9%, so it seems it’s still charging but very slowly.
Until it is resolved there is no knowing what it is, hardware or software.
The phone is under warranty for two years and it doesn’t seem as though you have done anything to void it so it is worth contacting support[at] fairphone dotcom and see what they have to say.
They can be a bit slow and ask you to do this and that but you should at least get an email acknowledging your request for support within a working day or two.
You will undoubtedly be asked to do a factory reset at some point so hopefully you don’t have important data on there that you can’t afford to loose.