This is pretty scant information but possibly there is a quick answer. Just guessing - you're running a secure WebFOCUS Reporting Server, right?
The error message is saying that MAINTAIN EXEC can't call into the WebFOCUS reporting subsystem to get data. Preview doesn't call MAINTAIN, it's just a report. The report can be gotten but MAINTAIN isn't getting permission to talk to WebFOCUS reporting (that's what EXEC does in MAINTAIN). MAINTAIN as configured generally uses the EDASERVE node as its communications hub.
If so, please make sure the EDASERVE node in your ODIN.CFG (the communications config file for your server) is using the same security mode as the main LOOPBACK node. You can get at this file from your server console - Workspace > Configuration, then from the TOC choose Configuration files > Communications.
If this is not the reason (notably, if you're running the WebFOCUS Reporting Server unsecured), then please provide complete PMF traces (in Settings, set Application Tracing to ON or ALL), re-run your load request, and provide a View Source file from your browser to Customer Support.
Bob Jude Ferrante
Director of Business and Development
WebFOCUS Performance Management
I'll take any questions about PMF - business or technical - anytime!