as seen on production many times:

first error message (more below)

Argument 1 passed to Piwik\Plugins\UsersManager\Model::getUsers() must be of the type array, null given, called in /plugins/UsersManager/API.php on line 351

URL: index.php?module=API&method=UsersManager.getUsers&format=Tsv&token_auth=XYZANONYMIZED&translateColumnNames=1&userLogins[]=&userLogins[]=