Requested form not found in web_submit_form

Hello

I'm new to Loadrunner and installed the 11.5 version and was trying to record a sap portal transaction..however I'm getting the following error in module web_submit_form when using the controller

Error -27979: Requested form not found

I recorded the script and performed the correlation and can successfully replay the script in vugen, but it always fails in controller...I'm just trying with 10 virtual users so it cannot be a resource bottleneck in the portal...

if I switch to URL recording everything wents fine but as far as I understood the HTTP based recording should be preferred.

Thanks and best regards

Michele