Undefined index.

Thanks for the report. This is probably due to a plugin or your theme doing something with language files that QM is not expecting.

* Are you able to deactivate your other plugins one by one until the error goes away and then let me know which plugin (and its version) is causing the problem? And try switching to one of the default themes such as Twenty Twenty in case your theme is the cause.
* Which language are you using?
* Which version of WordPress are you using?
* Which version of QM are you using?
* Is there anything unusual about the configuration of your site?

Thank you for your fast reply. I apologise for not giving you more details. When this notice appeared, I was in the middle of trying to sort out the cause of a plugin to fail with the Error: cURL error 28: Operation timed out after 5000 milliseconds with 0 bytes received (http_request_failed)

I also have persistent messages in the Site Health Status:

The REST API request failed due to an error.
Error: cURL error 28: Operation timed out after 10001 milliseconds with 0 bytes received (http_request_failed)

The loopback request to your site failed, this means features relying on them are not currently working as expected.
Error: cURL error 28: Operation timed out after 10001 milliseconds with 0 bytes received (http_request_failed)

The plugin started working again and I’m unsure of exactly why, other than some changes in PHP settings. The Site Health cURL errors still occur.

The above is probably not related to the notice and QM, and the notice has stopped appearing. Unless it does, I will hold on further investigations, but here are some answers to your questions:
WordPress Ver. 5.4.1-en_GB
Theme: Twenty Eleven
Query Monitor Ver. 3.6.0