Have an application with Deprag SFM Screwdriving. (and an UR10e robot arm)
We used also a Delta PLC and communicat with the screwdriving controler via Ethernet port.
We do not use the Digital I/O in the robot arm because the communication between the robot arm and the PLC is the Ethernet IP.
There are some cases when the robot goes into C314A0 SPI Critical error state.
We checked the I/O connection and everything was fine. The polyscope is the newest version.
We use external power supply (24V) not the inbuilt one.
Thank you,
Why not use the built-in 24V supply, if you’re not using the UR I/O? Just connect the ground of your external 24V supply with the UR 0V/GND, and you should have no issues communicating with the robot. 
It sounds like the external supply is failing at delivering a constant 24V for some reason.