When you exit an application that does not automatically mean it is ended in right that second. It only means it has hidden its windows. What you can try is disabling the plugin. Is the error gone? Then you can be pretty sure it is that dll. If the error is still present try disabling larger parts of your code. This way you can surround the error and might get a hint where that bug hides. If the error occurs randomly, probability that you are randomly overwriting something in the memory is pretty high. Could be bad set pointer e.g. Prepare for a tough battle... ;-)

Good luck!