Have you tried turning on logging from within the QlikView document to see if this provides any further information? This can be accessed through Settings > Documents > Gneral Tab and checking the Generate LogFile box.
From there, if you manually reload the document QlikView will put a log file into the same directory as your report and hopefully there will be some detailed information in this file as to why the reload fails.
Also, if you are updating the report via a batch mode it may be worth temporarily commenting out the line set errormode (if used in the script) as this will potentially hide any useful errors.
I hope this helps.