In order to avoid future errors in the authentication of users using external database authentication plugin, it is necessary to add an additional verification of the data type varchar in the field mapped for the username in the external database.
This test is located in http://MODDLESITE/m/stable_35/admin/category.php?category=authsettings (see the screenshots attached).
The current result of the test authentication settings:
The expected result:
Add an extra check on the data type of the username field in the external database table.