Since a few Weeks I coudn't add tags to my articles, there was a 500 error in the AJAX Request and the input field was readonly.

The error was a Memory exception >500MB, and can be solved by editing

core/components/articles/processors/extras/gettags.class.php

replace:

$articles = $parent->getMany('Children',array('deleted' => 0));
$articleIDs = array();
foreach($articles as $article){
    $articleIDs[] = $article->id;
}

width:

$templateVariable = $this->modx->getObject('modTemplateVar', array('name' => 'articlestags'));

This performes much better and the error is solved. I also created a pull request:

https://github.com/modxcms/Articles/pull/136