This is likely due to an excessively large audit record from when a user made a selection of many values in a specific field.
If you look at the bottom of the fullscan.messages.log file (in the profiles\default\QVX_LastRun folder), you will likely see something like the following. (Note that the -F entries signify failures.)
Multi-Transform Audit Log - OPERATOR-0016-F: toolId 219.0, name 'Multi-Transform Audit Log' - Exception 'BufferAllocatorException' occurred in the 'process' function for thread 0. (FullScan.Step_1)
Multi-Transform Audit Log - OUTPUT_BUFFER-0005-A: An output record is too large. The hard limit on output buffer size of 33,554,432 bytes has been exceeded by a request for 66,322,432 bytes. (FullScan.Step_1)
Multi-Transform Audit Log - OPERATOR-0005-F: toolId 219.0, name 'Multi-Transform Audit Log' - the 'process' function failed for thread 0. (FullScan.Step_1)
If this is the case, the fix is to find that large Audit log file, open it and find the huge entry and remove it. One way to search for it is to manually look at the log file (with word wrap on). Or, you could load all the audit log files into QlikView with something like the attached, and search for the long entries.
audit_log_loadAll.qvw 320.5 K