Seems I have the same problem, when I'm using the 'General CSV file', it seems endless, with just a small csv less than 600kB.
Here is the top 3 lines in my csv file.
1,2013-12-01 18:37:23,2013-12-01 18:39:58,Action
1,2013-12-01 18:37:23,2013-12-01 18:39:58,autosearch
And I have change the Case ID to be 'SessionID', Task ID to be 'Action', StartTime to be ‘StartTime', and End Time to 'EndTime'. Data Format to be 'Y:M:D H:M:S', I also change the format to 'Y-M-D H:M:S' to have a try. Both didn't work, could you tell me why?