Are you still on Monarch v8 Pro, as suggested by your profile? If it would help, I can check the export in v10.5 or v11 for you. Has the model worked in the past? If so, have there been any recent system changes?
Can you export to PDF from Monarch Pro on your desktop? If so, then the problem is on the server. Perhaps it's run out of disk space? Or there's a permission issue with the network resource it's trying to write to?