Change #1268566 had a related patch set uploaded (by Aude; author: Aude):

[mediawiki/extensions/ReadingLists@master] Update i18n message for the readinglists-db-error-not-own-list error

https://gerrit.wikimedia.org/r/1268566

Quick check-in: The current patch changes this to “You can’t access this page. You may be logged into a different account than the one in the URL.” Which is already an improvement.
URL is a bit jargony though and –even though wide-spread– expert knowledge, should it say "web address" instead? @Sneha

Change #1268566 merged by jenkins-bot:

[mediawiki/extensions/ReadingLists@master] Update i18n message for the readinglists-db-error-not-own-list error

https://gerrit.wikimedia.org/r/1268566

This 1-pointer should have also been on the sprint, as it already has a patch pending.

Checked on enwiki beta:

• the message is in place
• the message is displayed when a user tries to see another user reading list (see the steps from T411056)
  

Notes:

• the message readinglists-db-error-not-own-list is not yet in translation. https://translatewiki.net/wiki/Special:Translations?message=readinglists-db-error-not-own-list&namespace=8 show the only old copy of the message.

• still wating for @Sneha reply on:

In T422496#11794498, @Volker_E wrote:

Quick check-in: The current patch changes this to “You can’t access this page. You may be logged into a different account than the one in the URL.” Which is already an improvement.
URL is a bit jargony though and –even though wide-spread– expert knowledge, should it say "web address" instead? @Sneha

Agree.

Sorry just seeing this now. I remember talking about the copy in slack with @aude but not able to find the thread. Seems I have wireframes updated to this "To access this page, you’ll need to be logged into the associated account." and it might be the result of conversation we had on this. Does this copy sound okay?

@HFan-WMF @Sneha this doesn't sound resolved? If not, could you add to the Reader Experience Team or create a new ticket and resolve this one? It's currently on an archived board.

Moving back to "Ready for Sprint" to update the error message to Sneha's newest copy suggestion. Will tackle when sprint capacity allows.