Re: error message that came out of the blue

Appears you have/running version 3.4.x of Moodle.

Please go to http(s)://yourserver/admin/settings.php?section=mobilesettings and see how that's set.

cat ./moodle34/admin/tool/mobile/version.php

shows:

defined('MOODLE_INTERNAL') || die();
$plugin->version   = 2017111300; // The current plugin version (Date: YYYYMMDDXX).
$plugin->requires  = 2017110800; // Requires this Moodle version.
$plugin->component = 'tool_mobile'; // Full name of the plugin (used for diagnostics).
$plugin->dependencies = array(
    'webservice_rest' => 2017110800

So it's related to mobile.

'spirit of sharing', Ken