Internal Error - ptVUserVars->_hDfePE is unexpectedly NOT NULL

I've just recorded a Web-HTTP/HTML script in URL mode (LoadRunner 11.52 build 4391).

It automatically adds web_concurrent_start(NULL); and web_concurrent_end(NULL); calls.

But it seems that everytime that a web_url or any other "web_" call is outside a concurrent "section" it fails to replay with this error:

Fatal Error -26499: Internal Error - ptVUserVars->_hDfePE is unexpectedly NOT NULL after xfeLrwSrvNetTaskItemPrimaryCreate. [MsgId: MERR-26499]

I'm new to LoadRunner, so I'm not sure if I did something wrong or if this is intended by design. But if so, then the recorder should not create such situations. Also, this error message could be a lot clearer, I found it tough to figure this out.