widgets.php - 403 Access to this resource on the server is denied!

G'day,

I'm trying to add a widget (Flex Mag: Ad Widget) to add a widget to a widget area (Post/Page Sidebar Widget Area).

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Selecting the widget and entering the required info works as intended, however, when clicking save I'm immediately bounced to a 403 Forbidden page and nothing is saved.

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URL: https://www.fritzmag.com.au/wp-admin/widgets.php
Error: 403 Forbidden Access to this resource on the server is denied!

The only thing I can see that may be causing the error is Defender's 'Prevent PHP execution' security tweak.

I have tried reverting the fix, which doesn't seem to work, and deactivating Defender completely.

Also, when attempting to add the same ad code into Appearance / Flex Mag Options / Article Settings the save button perpetually spins and nothing is saved.

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I've tried all the standard using several browsers, computers, internet connections cache empty, disabling other plugins etc.

I've not noticed any other abnormalities across the site.

Thanks