Error: parsererror - SyntaxError: JSON.parse:
unexpected non-whitespace character after JSON data at line 2 column 1 of the JSON data
You may get this self-explanatory error at run-time if you specified a non-existing page item in the list of items to be returned after invoking a PL/SQL procedure from within a dynamic action in Oracle Application Express.
This may easily happen if you mistyped the page item name for instance, but in my case I had completely forgotten to create the item P4_ID.
Luckily I had an epiphany before going totally nuts.