I had the exact same issue with the symptoms you described on a 2023 Radcity Step thru. This issue happened a few weeks after purchase with under 20 miles on the ebike. A new controller got the Radcity back to normal. Fast forward to Aug 2024; I have an error 21 on the LCD and all the same symptoms as before. Rad sent me an seat post mounted external controller to replace the internal controller located in between the pedals.