Hi Simon,
Well, that is it.....we will have to go through all the lines 
Did you stop the logging when the error has occurred? or it was still enabled?
I have heard that SAP is going to improve the publication's log files for the dynamic users but this will be introduced in SAP BI4.2.
To reduce the log files you can sort them according to the time span when the error was encountered.
Sort the log files........This is the best we can do while playing with them.
~SwapnilY