155f fresh install
php: 7.1.24
OPC 2.0.5 munged from Git Hub

Created a product test order with OPC & Square was successful.
Dowloable pdf/zip file with OPC after clicking confirm the order, the curser just spins.
So no error logs were generated.
Turned off OPC to 3 page checkout and was successful.

Sent you a pm when you have time

Thanks,
Mark