Blank white page with JSON when saving block edits

Hi - I am have an issue in a site. After editing blocks on a page, when I click save, I get a new browser tab open, showing a blank white page with some json - something like:

{"message":null,"title":null,"time":"2017-08-10 09:04:29","redirectURL":"","aID":"21765","arHandle":"Main Light","bID":"21760","cID":"1770","pages":[{"name":"Page name","cID":"1770"}]}

You then just close the 'blank' page, and then refresh original page to see edits. The changes do get saved, but it's an annoying issue for admin staff when continually having to go through this process for every amendment made.

I am using a custom theme, and have checked extensively and there are no errors in the theme.

This issue has only been happening since upgrading to I am not able to upgrade to 5.8 at this point in time.

Has anyone seen this issue, or can suggest where I might start to look to fix?

NB: I have already seen this post about this issue:

and this bug fix:
(not relevant as amends are included in

Thanks all.

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pixelhero replied on at Permalink Reply
You will have javascript errors in your console I imagine when the page/edit screen loads.

Can you paste any console errors from your browser when loading the page and opening the edit box where you see the issue?

drbiskit replied on at Permalink Reply
Hi @pixelhero - Thanks for the response, but there are no errors showing at all. I've extensively checked for js errors, and even things like 404's on missing files, etc.

MrKDilkington replied on at Permalink Reply
Hi drbiskit,

When experiencing errors using a custom theme, it is recommended to activate the default Elemental theme to see if you still experience the issue.

This will help point you in the right direction of where the issue might be.
drbiskit replied on at Permalink Best Answer Reply
Hi - Thanks @MrKDilkington. Yes, I'd already done that - sorry should have mentioned! - It wasn't showing any php or js errors with the Elemental theme activated, but the saving issue described was still happening...

I have fixed it now, it turns out that it was being caused by 'Google Tag Manager' tracking code. I had to disable the code while in edit mode and it has fixed the issue.

<?php $c = Page::getCurrentPage(); if (!$c->isEditMode()) {
} ?>

It has meant that I've had to hard code rather than use the Dashboard -> Systems & settings -> SEO & Statistics -> Tracking Codes - Not ideal, but not the end of the world.