Because the island has bad code so it affects all the islands till you fix it.
What will confuse you is the error says portal-box-52.
But when you goto custom islands it may or may not have the same number.
That is why I said to post content from the php file.
To make sure we are dealing with the bad island.
Remember that the default tags need to be kept in place.
And php needs to be in a separate area from html code.
Because the error says line 4 I am assuming you placed the iframe in the php section instead of the html section.
Look at the sample text below.(included in v7.7.4 above where you make a entry).
If the error was in the html section it would be in around line 6 with this sample
If it was posted above the $body = <<<EOF or below the EOF; it would be incorrect.
Because it is a html tag
/* PHP CODE HERE, IF NECESSARY */
the php body of your custom box here
/* DO NOT CHANGE THE LINE BELOW. IT IS REQUIRED. */
$body = <<<EOF
the html body of your custom box here
/* DO NOT CHANGE THE LINE ABOVE. IT IS REQUIRED. */