Issue link: https://ci.moodle.org/job/W.03.01%20-%20Behat%20-%20Chrome%20+%20Postgres/635/

001 Scenario: Searching for a non-existing user          # /var/www/html/enrol/manual/tests/behat/quickenrolment.feature:130

      And I set the field "Select users" to "qwertyuiop" # /var/www/html/enrol/manual/tests/behat/quickenrolment.feature:133

        Exception: invalid argument: 'value' must be a string

          (Session info: chrome=79.0.3945.117)

          (Driver info: chromedriver=79.0.3945.36 (3582db32b33893869b8c1339e8f4d9ed1816f143-refs/branch-heads/3945@{#614}),platform=Linux 4.15.0-74-generic x86_64) in /var/www/html/vendor/instaclick/php-webdriver/lib/WebDriver/Exception.php:144

This has been detected to be happening at very least in the following scenarios:

• Searching for a non-existing user
• Teacher can search and enrol one particular student
• If there are less than 100 matching users, all are displayed for selection
• If there are more than 100 matching users, inform there are too many
• Change the Show user identity setting affects the enrolment pop-up : (happens 2x often than the others)
• Changing the Maximum users per page setting affects the enrolment pop-up

Never failing twice apparently, but the last one, that have happened twice a couple of times at very least.