In most cases when people write us about errors when using Webserver Stress Tool, they have discovered exactly what Webserver Stress Tool is designed to discover: Problems with scripts or webservers that crash or fail when multiple users access it simultaneously.
You can be quite sure that you have a server problem if you get errors other than "timeout" and "404". For example the error code 500 (internal server error) is never generated by Webserver Stress Tool but by the webserver.
Here are some tips:
Run the test with just one user, increase the number of users step by step. If that works with one or a few users without errors it is almost certain that your website has a problem with simultaneous users
Escalate this approach using a ramp test
Enable Write Detailed and User Logs and Write Headers to Log and review the user logfiles for hints about the problem
Enable Save All HTML files and review the resulting webpages
Review the webserver's or the application's error log file
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.