I have experienced this in my start as an Admin in Sugar CRM (v9).
Hope the solution helps others...
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Clicking Export in a report results in this:
"Database failure. Please refer to sugarcrm.log for details."
(I only really used Summation reports so far)
But if you add a 'Count' column to your report (in Choose Display Summaries) then it fixes it
and the report exports/downloads :-)