This KB explains why the Send to Proof Server task fails with the message: "Error message: Cannot write output file".
When using the "v20-1_P75x0CT-1200x1200_ProofingPrintMode.epl" file, the Send to Proof Server task fails with a message "Error message: Cannot write output file".
Starting from Pack Proof 21.07, there is a new Proof Print Mode to improve the sharpness on the Epson SC-Pxx5x0 devices (e.g. SC-P7500). When using this EPL, the latest Epson firmware must be installed with the additional hotfix for the Esko Proof Server. If the hotfix is not installed (available on the IEP DVD in the \\Extra\Epson_SC_Px5x0_Extra_Proofing_Print_Mode folder,) you will get the "Cannot write output file" message.
If the error appears even when the hotfix is installed, then it might be a disk space issue.