screen-scraper server model goes wrong on linux pc

We cannot find the original source of an exception.
When screen-scraper is invoked from the commandline for adding a task,
an exception is caught by an interpreted java script that we are calling.
The exception is not very descriptive:

java.lang.ClassNotFoundException: com.mysql.jdbc.Driver

It would at first glance appear that CLASSPATH is not set, but it is:

# CLASSPATH=$HOME/ss/mysql-connector-java-5.1.5-bin.jar:$CLASSPATH java -jar screen-scraper.jar -s "generic - call-script" -p "scrapeFile=foo.quote"

if we inspect the jar we can also conclude that the driver indeed does exist:

# ls -la com/mysql/jdbc/Driver.class
-rw-r--r-- 1 foo foo 692 2007-10-05 21:08 com/mysql/jdbc/Driver.class

This is obviously not a lot of information, but with some luck maybe someone has
experienced similar issues. Any information would be greatly appreciated.