Hello,
I had the exact same problem. My description as well: From one day to the other I could not charge the FP3. When running and connected to power USB the LED lit up but the charging symbol of the battery in the display (top right) did not appear and it did not charge. When connected and turned off, the LED lit up and it “booted up” vibrating, showing Fairphone logo and a message that it ‘charges, do not disconnect’ then going blank, vibrating, showing logo, message and going blank… In the end the phone would actually lose battery life quickly due to this behaviour and not charging at all.
In short it was solved by a replaced bottom module. I tried hell a lot of cables and cleaning the USB port, freezing the battery and what not. Yet as soon as I replaced the bottom module it charged again.
Note: I also ordered a new battery together with the bottom module, which I tested first. It showed the same message with 67% charged but also looped and didn’t charge at all. I used the battery to sync and copy some data before replacing hte bottom module
Hope this information is handy for some other costumer, I like the fact that the phone is repairable and I could make use of it. I even replaced a malfunctioning camera a year ago as well.
best wishes
Stefan