I have the same issue..

Using 2 rx 570 directly to the board, all fines.

Adding 1 card using a riser.. and

openCL error -61 - cannot allocate big buffer for DAG. Check readme.txt for possible solutions
opencl error -38 - cannot write buffer for dag

I uninstalled the driver.. put 16gb pagefile, reinstalled drivers, repatch.. nothing works

I removed the card plugged with the pci riser and still the same error.. I am confused

any idea?!?