You might see this error in your .tracesql logs.
Apparently this happens if you are performing page transfers or pop ups (modal windows) and you are using ApiObject variables (search for ID 652536.1 in My Oracle Support for further details).
In my case, I saw this error after code for calling and using a component interface. The solution that worked for me was to set the value of all ApiObject variables I was using to null once I was finished with them (e.g. after saving the CI).