Scott is correct. The issue has been resolved in our most recent releases of Cloverleaf.
The limit was on Windows based systems. The limit is 128 total threads per server. Let me clarify the way these are counted.
Each thread counts as one user. These include command and xlate threads too. So if you had a process with 10 protocol threads your total would be 12. Add these up among all your sites and see if you exceed this amount.
This was resolved as of 5.2.1 Rev 2.