error when comparing two variables

["K. A. Bouton" <k.a.bouton@xxxxxxxxxxxxx>]

My LAS server (v6.1) is up and running fine (it seems).
http://www.cgam.nerc.ac.uk:8080/las/servlets/dataset

I am using a .des file to display 4 datasets as one. Looking at any
individual variable from the dataset works fine.
But when I try to compare two - say xaafc,temperature,November to
xaafc,temperature,January - I get the following error

javax.servlet.ServletException
	at gov.noaa.pmel.tmap.FormFilter.doFilter(FormFilter.java:95)
	at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Application
FilterChain.java:213)
	at
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterCh
ain.java:193)
.....lots more
	at java.lang.Thread.run(Thread.java:536)

Root cause of servlet exception was:

javax.servlet.ServletException
	at
gov.noaa.pmel.tmap.TemplateServlet.error(TemplateServlet.java:123)
	at
org.apache.velocity.servlet.VelocityServlet.doRequest(VelocityServlet.java:3
63)
	at
org.apache.velocity.servlet.VelocityServlet.doGet(VelocityServlet.java:294)
.....lots more
Root cause of servlet exception was:

javax.servlet.ServletException
	at
gov.noaa.pmel.tmap.TemplateServlet.handleRequest(TemplateServlet.java:183)
	at
org.apache.velocity.servlet.VelocityServlet.doRequest(VelocityServlet.java:3
33)
	at
org.apache.velocity.servlet.VelocityServlet.doGet(VelocityServlet.java:294)
	at
org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Application
FilterChain.java:213)
.....lots more
	at org.apache

Do datasets have to be different - or can you compare within datasets?
Otherwise - what else might be wrong?

thanks
Katherine