This extension isn't working on 1.35.1, probably since the 1.35 release.
Topic on Extension talk:ExternalRedirect
As described in this thread, this is caused by a deprecated method in the extension.
Therefore, you simply have to go to ExternalRedirect/ExternalRedirect.php and edit line 55.
function wfExternalRedirectRender($parser, $url = '') {
global $wgExternalRedirectNsIDs, $wgExternalRedirectPages, $wgExternalRedirectDeniedShowURL;
// $parser->disableCache();
$parser->getOutput()->updateCacheExpiry(0); //fix for wiki 1.35
if(!wfParseUrl($url) || strpos($url, chr(13))!==false || strpos($url, chr(10))!==false) {
return wfMessage('externalredirect-invalidurl')->text();
}
if(in_array($parser->getTitle()->getNamespace(), $wgExternalRedirectNsIDs, true)
|| in_array($parser->getTitle()->getPrefixedText(), $wgExternalRedirectPages, true)) {
header('Location: '.$url);
return wfMessage('externalredirect-text', $url)->text();
} else {
return wfMessage('externalredirect-denied')->text().($wgExternalRedirectDeniedShowURL
? ' '.wfMessage('externalredirect-denied-url', $url)->text() : "");
}
}
Hope that helps!
Cheers
ExternalRedirect v1.2.0, released on January 3 2022, supports MediaWiki 1.35+