I had the same. Looking into the logs i came across this error
6/10/14 9:47 AM Europe/Amsterdam Starting job type: Export
PHP Error[8]: unserialize(): Error at offset 65528 of 65535 bytes
in file /var/www/zurmo/app/protected/modules/export/jobs/ExportJob.php at line 141
#0 /var/www/zurmo/app/protected/modules/export/jobs/ExportJob.php(141): unserialize()
#1 /var/www/zurmo/app/protected/modules/export/jobs/ExportJob.php(107): ExportJob->processExportItem()
I expect because i have added custom fields, but i'm not sure. I used the SQL query generated from the report function and exported it to manually.