Back-end login no longer possible (FIXED)

I have upgraded to R3.0.3 and frontend and back-end worked fine without any errors. The next day the site is still running, but I can’t log into the backend. It throws the following error:

Warning: in_array() expects parameter 2 to be array, null given in public_html/system/modules/core/library/Contao/System.php on line 110
#0 [internal function]: __error(2, 'in_array() expe...', '/home/eu164060/...', 110, Array)
#1 /public_html/system/modules/core/library/Contao/System.php(110): in_array('getInstance', NULL)
#2 /public_html/contao/main.php(147): Contao\System->import('TaskMessages')
#3 /public_html/contao/main.php(115): Main->welcomeScreen()
#4 /public_html/contao/main.php(319): Main->run()
#5 {main}
Fatal error: Class 'TaskMessages' not found in /public_html/system/modules/core/library/Contao/System.php on line 110

Below a section of System.php code:

public function __get($strKey)
{
if (!isset($this->arrObjects[$strKey]))
{
if ($strKey == 'Input' || $strKey == 'Environment')
{
$this->arrObjects[$strKey] = $strKey::getInstance();
}
else
{
return null;
}
}

return $this->arrObjects[$strKey];
}

/**
* Import a library and make it accessible by its name or an optional key
*
* @param string $strClass The class name
* @param string $strKey An optional key to store the object under
* @param boolean $blnForce If true, existing objects will be overridden
*/
protected function import($strClass, $strKey=null, $blnForce=false)
{
$strKey = $strKey ?: $strClass;

if ($blnForce || !isset($this->arrObjects[$strKey]))
{
$this->arrObjects[$strKey] = (in_array('getInstance', get_class_methods($strClass))) ? call_user_func(array($strClass, 'getInstance')) : new $strClass();
}
}

I did run the Contao check on the site, it gave no errors.
I checked the install script; it gave no errors.
I entered an incorrect password, it properly detects this.
I used phpadmin, database showed no issues.
I have a second (test)site on the same platform and database; never had any issues.
Without access to the backend the situation is quite......

I had the same problem and started this thread
viewtopic.php?f=9&t=5956

the answer was to disable the 'tasks' centre in settings and other areas. Been working fine ever since.

Thanks for the tip.

Changing $GLOBALS['TL_CONFIG']['coreOnlyMode'] = true; in default.php allowed me to logon again as a temporarily solution.

I tried to follow up on you tip to skip the tasks module, but cold not find the .skip file anywere. Therefore installed the " tasks" extension; after that everything worked fine.

Also noted that the localconfig did not seem to be fully up to date, eg - it still mentioned version 2.11.8 where version 3.0.3 is installed. Have created a new file from a fully functional "new" site. But this did not make any difference.