Whilst comparing the production and test server I did overlook one "small" thing. I had recently updated the porting set from 6.1c.13 to the new 8.8a version.
So I rolled back the porting set on test and managed to replicate my issue. So following your suggestions I set the SLMHOME, CLASSPATH and LD_LIBRARY_PATH at the start of the users crontab. Scripts ran by cron now work.
Thanks for your input on this issue.