A javascript error has occured: Session Error

I'm getting the following error:
"A javascript error has occured: Session Error.
The fulll stacktrace for this error can usually be found in your server's error log."
I've also been able to deduce it's related to a search function, given when I visit http://127.0.0.1/:8080/openmrs/dictionary/ an try and search the database, this pops up. It also shows up when trying to find patients, and in the cohort builder.

This is OpenMRS 1.6.1 using Tomcat 7.0 & MySQL5.1 on Windows 7.

I managed to deduce that the C:\...\Tomcat 7.0\logs\service-stderr.xxx.log file and catalina.xxx.log have the following lines being written to them when this happen:
org.apache.timcat.util.http.Parameters processParameters
WARNING: Parameters: Invalid chunk '' ignored.

I did a few searches in the wiki but didn't come up with a javascript error related article. Nothing from google that is related to OpenMRS. How can I track this error down and fix it? It's rather debilitating to the system.

Many thanks,
Rich