The previous answers are incorrect. Mahout 0.8 had a MapReduce version of seqdirectory which was a new feature. A bug in the MR version was causing the exception you are seeing.
To execute seqdirectory with Mahout 0.8, please use the sequential version by specifying the -xm sequential option to your command line.
mahout seqdirectory -i inputFolder -o outputFolder -xm sequential
By default seqdirectory executes the MR version if none specified.
This issue has since been fixed in Mahout 0.9.