My CakePHP Version: 2.8.4 PHP: 7.1.12 (FastCGI)
I just noticed that the error log is full of this:
2018-01-07 09:05:33 Warning: Warning (2): Narrowing occurred during type inference. Please file a bug report on bugs.php.net in [/var/www/******/httpdocs/lib/Cake/Core/App.php, line 547]
Trace:
include - CORE/Cake/Core/App.php, line 547
App::load() - CORE/Cake/Core/App.php, line 547
spl_autoload_call - [internal], line ??
TranslateBehavior::_getLocale() - CORE/Cake/Model/Behavior/TranslateBehavior.php, line 534
TranslateBehavior::beforeFind() - CORE/Cake/Model/Behavior/TranslateBehavior.php, line 104
ObjectCollection::trigger() - CORE/Cake/Utility/ObjectCollection.php, line 128
CakeEventManager::dispatch() - CORE/Cake/Event/CakeEventManager.php, line 243
Model::buildQuery() - CORE/Cake/Model/Model.php, line 3084
Model::find() - CORE/Cake/Model/Model.php, line 2998
DboSource::query() - CORE/Cake/Model/Datasource/DboSource.php, line 595
Model::__call() - CORE/Cake/Model/Model.php, line 827
GalleryPicturesController::getPicture() - APP/Controller/GalleryPicturesController.php, line 35
ReflectionMethod::invokeArgs() - [internal], line ??
Controller::invokeAction() - CORE/Cake/Controller/Controller.php, line 491
Dispatcher::_invoke() - CORE/Cake/Routing/Dispatcher.php, line 193
Dispatcher::dispatch() - CORE/Cake/Routing/Dispatcher.php, line 167
[main] - APP/webroot/index.php, line 110
There are plenty of such an error.
Not just about GalleryPicturesController.php, but always refers to "Cake/Core/App.php, line 547".
What can be the cause of the error?
UPDATE
Solution:
class AppModel extends Model
{
public $locale = 'hun';//I just added it
But now:
2018-01-07 11:27:20 Warning: Warning (2): Narrowing occurred during type inference. Please file a bug report on bugs.php.net in [/var/www/******/httpdocs/lib/Cake/Core/App.php, line 547]
Trace:
include - CORE/Cake/Core/App.php, line 547
App::load() - CORE/Cake/Core/App.php, line 547
spl_autoload_call - [internal], line ??
__ - CORE/Cake/basics.php, line 587
include - APP/View/GalleryPictures/open.ctp, line 64
View::_evaluate() - CORE/Cake/View/View.php, line 971
View::_render() - CORE/Cake/View/View.php, line 933
View::render() - CORE/Cake/View/View.php, line 473
Controller::render() - CORE/Cake/Controller/Controller.php, line 963
Dispatcher::_invoke() - CORE/Cake/Routing/Dispatcher.php, line 200
Dispatcher::dispatch() - CORE/Cake/Routing/Dispatcher.php, line 167
[main] - APP/webroot/index.php, line 110
In: GalleryPictures/open.ctp, line 64:
<th><?php echo(__('Feltöltő')); ?></th>

As already mentioned in the comments, that is a bug in PHP, and you are using the PHP version where that bug apparently was introduced, that is 7.1.12, see the related bug reports:
The bug was fixed in in PHP 7.1.13 and 7.2.1, so I'd suggest that you upgrade your PHP installation, or if you cannot upgrade (or downgrade for that matter), try disabling opcode caching, which is where this bug seems to stem from.
If upgrading doesn't fix the problem, report it over at https://bugs.php.net