It seems some process is taking too much time to execute and cannot get completed within the default timeout period. Are you uploading a large file or something? By default the timeout is 2 minutes.
You may try increasing the timeout period in your
web.config
file and check if that works.
<system.web>
<httpRuntime executionTimeout="240" />
</system.web>
Here 240 is the no. of seconds an ASP.NET page is given before the operation is assumed to have failed and the page terminated. So I have changed the execution time-out to 4 minutes.