Many, many thanks to the OP for starting this thread and persevering to a solution. I recently obtained an APC SMT1500C and was experiencing the same issues. This thread saved me countless hours of troubleshooting. I could not find the cable indicated (AP940-0625A) at a reasonable price but I did find another cable that works, AP940-1525A, at a price I could stomach. The cables are equivalent, the only difference being that the AP940-1525A is 15 feet versus 6 feet for the AP940-0625A. So this provides an alternative for anyone having trouble finding the cables. I can confirm that the AP940-1525A is working in my setup.
Thanks again
D