Hi Peter,
Check your MAXSCHEDSESSION value (Q STATUS shows the current limit). If it's to 
low you will receive these kind of errors. Use the SET MAXSCHEDSESSIONS 
schedulepercent command to update this value. If server workload is not an 
issue in your environment set this percentage to 100% (so SET MAXSCHEDSESSIONS 
100)
Also check the MAXSESSIONS paramater in your server's option file. The 
percentage is relative to this value.
Kindest regards,
Eric van Loon