XF 1.5 Create Thread / Edit admin area - nothing happens but date timestamp

[PJK]

Yesterday I was editing one of the widgets, and when I clicked save the page would reload with a blank white page and a time stamp at the top with the date and time. Today a user said they tried to a create a thread and the same issue happened:
http://imgur.com/a/reTPt

I'm unsure how often this is happening or why it is happening, nor when it started. First I noticed it was yesterday. How can I troubleshoot this?

Thanks.

 

[PJK]

I'm editing the widget here:
https://www.dropbox.com/s/givqfse03878l5l/Screenshot 2017-05-21 18.06.20.png?dl=0

Once I click Save Widget, this comes up:
https://www.dropbox.com/s/5geep0vti6w0l8n/Screenshot 2017-05-21 18.07.00.png?dl=0

Any ideas what is causing this and how I can fix it so I can update this widget? Nothing actually saves in the widget as this is happening.

Thanks.

 

[Paul B]

https://xenforo.com/community/threads/where-to-obtain-support-for-third-party-add-ons.92672/

 

[PJK]

https://xenforo.com/community/threads/where-to-obtain-support-for-third-party-add-ons.92672/

Thanks. Considering this happened when editing the widget and when a random user tried to create a thread, I assumed it wasn't isolated to that plugin. Do you think the List Widgets plugin is causing the issue with Create Thread and with when I try to edit a widget?

 

[PJK]

I suspect this issue is actually not related to that Widget plugin. The user who reported the same error when creating a thread said when they changed the text in the post, the error went away. But if they post with the same content as before, that error keeps happening.

What's the best way to troubleshoot this? Any advice is appreciated.
Thanks.

 

[PJK]

https://xenforo.com/community/threads/where-to-obtain-support-for-third-party-add-ons.92672/

I've disabled all addons using $config['enableListeners'] = false; in config.php, and the issue still happens:
https://www.dropbox.com/s/9ji7h5ev4pz1diu/Screenshot 2017-08-09 17.10.46.png?dl=0

What can I do to troubleshoot this issue? Cheers!