Sorry for the late reply.
Did you customize the theme main code? The notice that you have shared is not related to the theme code.
Also please check deactivating all the plugins on by one and let us know.
No – I have not modified the Attitude theme itself – only via a child theme.
However, I am glad to report that this issue went away on its own. I don’t know what it was about or what fixed it but I have not seen it for some good while, so I propose we close this request.
Apologies for troubling you if it was not a theme issue – it did look like it the time.
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