I was Facing the same issue Crystal report version 11.5. After a lot of search I found different soluton like "HKEY_LOCAL_MACHINE\SOFTWARE\SAP BusinessObjects\Crystal Reports for .NET Framework 4.0\Report Application Server\InprocServer increase limit here etc etc but didn't work. Then I just dispose my reprtdic properly problem fixed. In my case after printing/gerenation report I did this
reportDocument.Close();
reportDocument.Clone();
reportDocument.Dispose();
reportDocument = null;
GC.Collect();
GC.WaitForPendingFinalizers();
It works for me hopefully it will help you also.:)