CUPS Max clients reached... after upgrade from Centos 5.5 to Rhel 6.1

I've upgraded a customer from Centos 5.5 i386 to RHEL 6.1 x86_64 and since then I'm frequently getting CUPS's "Max clients reached, holding new connections..."

The number of users is well below 100 ( normally 30-40 ) and the workload is the same.

Any suggestion?

best regards,

LF