Looks like this bug, which should have been reported in the plugin JIRA:
http://jira.codehaus.org/browse/GRAILS-4760If the fix specified in the description doesn't work you can use the older deprecated method of specifying job configurations, i.e.
String cronExpression = "0 0 6 ? * MON,WED,FRI" // every monday, wednesday, friday at 6am
That's what we're doing until the bug is fixed. It logs deprecation warnings at startup but works fine.
Burt