Please try a different charger. Make sure it outputs at least about 1.0 A (1000 mA).
Here is a list of working ones.
I had this problem with a 750 mA charger after encrypting my phone, 800 mA and 1000 mA chargers solved this.
P.S.: This is a community forum. Fairphone employees might drop by occasionally, but if you want to contact official Fairphone support, here’s a link.