Have you tried to run a trace and look at your server logs when you execute your script? More than likely an configuration issue. Sorry if this is not much help but that is all i can think of at present
I have seen this error coming when my application servers were down and I run the loadtest on it.Sometimes I have seen this error when Load model is given was not right,iterating login and logout everytime.
you need to check the true logs for checking whether request is passing or failing.
So make sure that you post enough information as how many users you are running,what load model,do you have think time,are you iterating logins everytime etc.
scripts : 4
workload : increasing
simulation time : 30 min
thinktime : 30 sec (to reduce server load, since some of the scripts creates some processes in the server)
1st 15 min all the users will loading with the time interval of 5 min..
last 15 min is the steady time
I have iterated the login and logout everytime since i thought breaking the transactions could produce some errors.