Overlay issue - blank screen with modx.window.js?

Getting an overlay issue where the screen is covered by a white overlay at seemingly random intervals.

Console shows:

Failed to load resource: the server responded with a status of 404 (Not Found)



Uncaught TypeError: this.mask.addClass is not a function
at S.onShow (modx.window.js:60)
at S.afterShow (ext-all.js:21)
at S.show (ext-all.js:21)
at Object.show (ext-all.js:21)
at MODx.onAjaxException (modx.js?v=ecdc0adb:122)
at h.Event.fire (ext-all.js:21)
at Ext.data.Connection.fireEvent (ext-all.js:21)
at Ext.data.Connection.handleFailure (ext-all.js:21)
at f (ext-base.js:21)
at m (ext-base.js:21)

Ran upgrade to 2.7.3 to see if a file was bad, but no change.

Anyone else experience this issue or have recommendations on troubleshooting further?