Unable to edit templates - console messages and modsecurity error

[TheLaw]

XF 2.3.x has added many security additions and wonder whether this issue is related. I'm simply trying to add a user group variable to a template. Should be accepted without issue, just one number and a comma. But I get an error saving.

Oops! We ran into some problems. Please try again later. More error details may be in the browser console.

I look in console:

lockdown-install.js:1 Removing unpermitted intrinsics
admin.php:1 Unchecked runtime.lastError: The message port closed before...
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Unable to edit templates - console messages and modsecurity error

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