Hi Christian,
you posted that your System characteristics changed : "this has been removed and replaced with a java only portal (so no JC01 but J01 )".
I had this issue several times, too, especially older Java-based Systems being upgraded "into this new instance naming convention".
To resolve this issue, check you landscape Parameters in SOLMAN_SETUP. There you can Change "JC" to "J".
Regards,
Dirk Osburg
