Sanitize Java Action Error

Hi,

We have recently upgraded a project from Mendix 6.10.7 to 7.3 and we are now finding the following error, 

Error in execution of monitored action 'executeaction' (execution id: 21474f30b5aed40, execution type: CLIENT).
 NoClassDefFoundError occurred for class 'org/apache/xml/serialize/HTMLSerializer'. Please make sure the userlib folder of
 your project includes the correct library which contains this class.

Has anyone come across this issue before?