One key item missing is the eof tags that should be in the portal box and preserved.
From ubb 7.6
]/* PHP CODE HERE, IF NECESSARY */
/* DO NOT CHANGE THE LINE BELOW */
$body = <<<EOF
Body of the custom box here
EOF;
/* DO NOT CHANGE THE LINE ABOVE */
Noted in red should not be deleted or edited.
Also note as I recall extra blank line feeds outside of the EOF tags could also cause the issue or break the portal island.
Check the
WIKI for examples
You may need to restore the EOF tag lines on existing custom islands you have edited.
Last but not least a complete copy of your portal box code would help
