I don't think this is related to the experiment - I just created a new account on beta enwiki (not via the account creation CTA dialog), then tried to save an article, and saw the same error.

What's happening

The following happens when you create an account:

• You're taken back to whatever page you were on
• The onSkinTemplateNavigation__Universal handler in ReadingLists runs, which looks for a default list ID. If that's null, it runs ReadingListRepository::setupForUser(), which takes a param, $silent . This method first checks for a default list - if one exists, it either returns the default list or throws the exception we're seeing in this bug report.

// Bypass the setup process if the default list already exists
if ( $defaultListId !== false ) {
    // Older code may expect the exception behavior, so only return if $silent = true
    if ( $silent ) {
        return $this->selectValidList( $defaultListId, IDBAccessObject::READ_LATEST );
    }

    throw new ReadingListRepositoryException( 'readinglists-db-error-already-set-up' );
}

• So, now the user has a default reading list. However, if they click the bookmark button immediately (without loading a new page or reloading this one), the bookmark button element will not have a data attribute with the default list id yet. Therefore, on click, it will make an API call that runs ReadingListRepository::setupForUser() with $silent = false.

Scope

I can reproduce this locally and on enwiki beta. I have not reproduced it on production enwiki. I've gone back in time locally and this issue has existed for at least a month, so I'm not sure why it's not failing on production.

What to do

A simple solution would be to ensure that ReadingListRepository::setupForUser() runs with $silent = true for ApiReadingListsSetup. Based on the comment in the code snippet above, I'm not sure why this wasn't done initially, but it seems like we should just return the default list instead of throwing an exception when a default list is found on setup.

Change #1282996 had a related patch set uploaded (by Anne Tomasevich; author: Anne Tomasevich):

[mediawiki/extensions/ReadingLists@master] Return default list from setup API if it exists

https://gerrit.wikimedia.org/r/1282996

Change #1282996 merged by jenkins-bot:

[mediawiki/extensions/ReadingLists@master] Return default list from setup API if it exists

https://gerrit.wikimedia.org/r/1282996

Thank you, @AnneT! Checked in enwiki beta - the bug is fixed.

Checked the cases

• with ?mpo=account-creation-reading-list-cta:treatment and with control
• in incognito window
• with cleared browsing data
• a temp user when a temp user created an account after publishing an edit