Do not pass any default TimeOut property to ConvertAPI and set HTTP Client timeout to 1800 seconds. The idea is to use the default converter timeout of ConvertAPI if no timeout is set and to prevent HTTP Client request deadlock and set the default HTTPClient timeout to 1800 seconds.
If, however, timeout is set we handle it as before, pass a timeout to ConvertAPI and set HTTPClient Timeout to: ConvertAPITimeOut+conversion_timeout_delta
conversion_timeout_delta = 10