Hello Manjiri,
I cannot say anything for certain the root cause but here's something you can try...
Its possible that your source response time is greater than that of your workbench timeout, so on the workbench in the Menu bar go to Help --> Settings --> Look for "Connection Timeout" & "Read Timeout" usually by default it will be set to 300 increase it to a reasonable number say 3000.
Now try your transaction with this change and if this works then look for an opportunity to improve the efficiency of your BLS for faster execution times or look for any possible bottlenecks or unnecessary massive data reads.
Hope this helps!
Regards,
Adarsh