Thank you very much for your reply! I think I’ll replace the board as soon as I can. I’ve still been trying to get things running on the LowRider using a PC with Repetier-Host, but when I send an M122 command through Pronterface I get the output below — which seems to indicate that the TMC UART communication isn’t working at all.
On top of that, my Z axis keeps slipping down when trying to draw the crown, so everything is a bit of a mess right now. At this point I’m pretty sure the issues are at least partly related to the SKR 1.4 board.
Thank you again for your help! 
SENDING:M122
T3 X Y Y2 Z Z2
Address 0 0 0 0 0
Enabled false false false false false
Set current 900 900 900 900 900
RMS current 1602 1602 1602 1602 1602
MAX current 2259 2259 2259 2259 2259
Run current 28/31 28/31 28/31 28/31 28/31
Hold current 22/31 22/31 22/31 22/31 22/31
CS actual 0/31 0/31 0/31 0/31 0/31
PWM scale
vsense 0=.325 ...
stealthChop false false false false false
msteps 256 256 256 256 256
interp false false false false false
tstep 0 0 0 0 0
...
Driver registers:
X 0x00:00:00:00 Bad response!
Y 0x00:00:00:00 Bad response!
Y2 0x00:00:00:00 Bad response!
Z 0x00:00:00:00 Bad response!
Z2 0x00:00:00:00 Bad response!
Testing X connection... Error: All LOW
Testing Y connection... Error: All LOW
Testing Y2 connection... Error: All LOW
Testing Z connection... Error: All LOW
Testing Z2 connection... Error: All LOW