CakePHP Internal Server Error: Undefined class constant 'ATOM'

Hello everyone, could you tell me, please, what could be the issue here?
I suddenly started having some problems with my CakePHP installation. First the debugger frame at the bottom-right corner of the page stopped working. I did F-12 (inspect element) on my system to find out why, I found the following error, which I suspect could be the reason:

**Undefined class constant 'ATOM'** An Internal Server Error Occurred.

Consequently when I try to do ‘print_r($varname)’ CakePHP throws the following error:

Error: __debuginfo() must return an array
File C:\xampp\htdocs\cakeapp\src\Controller\UsersController.php
Line: 437 

Then when I try to set view variables using like so: $this->set('variable', $variable); I get the following error:

Error: Uncaught Error: Undefined class constant 'ATOM' in C:\xampp\htdocs\cakeapp\vendor\cakephp\chronos\src\Traits\FormattingTrait.php:133 Stack trace: #0 C:\xampp\htdocs\cakeapp\vendor\cakephp\cakephp\src\I18n\DateFormatTrait.php(431): Cake\Chronos\Chronos->toIso8601String() #1 C:\xampp\htdocs\cakeapp\vendor\cakephp\cakephp\src\Error\Debugger.php(606): Cake\I18n\FrozenTime->__debugInfo() #2 C:\xampp\htdocs\cakeapp\vendor\cakephp\cakephp\src\Error\Debugger.php(530): Cake\Error\Debugger::_object(Object(Cake\I18n\FrozenTime), 3, 1) #3 C:\xampp\htdocs\cakeapp\vendor\cakephp\cakephp\src\Error\Debugger.php(495): Cake\Error\Debugger::_export(Object(Cake\I18n\FrozenTime), 4, 0) #4 C:\xampp\htdocs\cakeapp\vendor\cakephp\cakephp\src\Template\Element\exception_stack_trace.ctp(37): Cake\Error\Debugger::exportVar(Object(Cake\I18n\FrozenTime), 4) #5 C:\xampp\htdocs\cakeapp\vendor\cakephp\cakephp\src\View\View.php(1168): include('C:\\xampp\\htdocs...') #6 C:\xampp\htdocs\cakeapp\vendor\cakephp\cakephp\src\View\View.php(1129): Ca
File C:\xampp\htdocs\cakeapp\vendor\cakephp\chronos\src\Traits\FormattingTrait.php
Line: 133 

In order to ensure that the errors wasn’t a result of some blunder I may have made in my code, I decided to install a fresh copy of CakePHP using composer, but the same thing is happening. What could possibly be the problem?