In my new Apex App, I created a page with a button to a PL/SQL process calling xlib_jasperreports.show_report. The code was copy/pasted from a well working application, but in the new app it created an error:
1 error has occurred
Error: SyntaxError: Unexpected token % in JSON at position 0
After hours of seaching, re-writing code and diff'ing the two apps, I found the pitfall: the newly created APEX page has the default setting "Advanced/Reload on Submit: Only for success". This had to be changed to "Always" on the page using JRI calls in the processing section.
Thought I'd share this point and possibly saving time for other developers.