magento-lts: WYSIWYG editor doesn't open on product admin page

Under the admin UI’s Catalog > Manage Products > Edit page, the WYSIWYG Editor for the Description or Short Description fields on the General tab doesn’t open. Instead I see a series of errors in my browser console (not sure which one’s are related to this specifically)

SyntaxError: Unexpected token ‘;’ (this is from a javascript snippet where var LOADING_TIMEOUT = ;) ReferenceError: Can’t find variable: BLANK_IMG (this is from a javascript snippet where Ext.BLANK_IMAGE.URL = BLANK_IMG;) Failed to load resource: the server responded with a status of 404 (Not Found) http://extjs.com/s.gif Can’t find variable FORM_KEY (I think this is the error that pops up when actually trying to open the editor)

I can reproduce this only after upgrading my Openmage 19.4.17 site to 19.4.18.