Missing Connection Exception Error

I am unable to access my MISP GUI or with the help of PyMISP API, I am getting the following error when trying to do so:

Error: An Internal Error Has Occurred. Please try your action again. If the problem persists, please contact administrator.

This is my error.log message:

Error: [MissingConnectionException] Database connection "Mysql" is missing, or could not be created.
Exception Attributes: array (
'class' => 'Mysql',
'message' => 'SQLSTATE[HY000] [1045] Access denied for user 'misp'@'localhost' (using password: YES)',
'enabled' => true,
)

This is my debug.log message:
Notice: Notice (8): Undefined variable: debugMode in [/var/www/MISP/app/View/Elements/footer.ctp, line 2]
Trace:
ErrorHandler::handleError() - APP/Lib/cakephp/lib/Cake/Error/ErrorHandler.php, line 230
include - APP/View/Elements/footer.ctp, line 2
View::_evaluate() - APP/Lib/cakephp/lib/Cake/View/View.php, line 971
View::_render() - APP/Lib/cakephp/lib/Cake/View/View.php, line 933
View::_renderElement() - APP/Lib/cakephp/lib/Cake/View/View.php, line 1224
View::element() - APP/Lib/cakephp/lib/Cake/View/View.php, line 418
include - APP/View/Layouts/default.ctp, line 80
View::_evaluate() - APP/Lib/cakephp/lib/Cake/View/View.php, line 971
View::_render() - APP/Lib/cakephp/lib/Cake/View/View.php, line 933
View::renderLayout() - APP/Lib/cakephp/lib/Cake/View/View.php, line 546
View::render() - APP/Lib/cakephp/lib/Cake/View/View.php, line 481
Controller::render() - APP/Lib/cakephp/lib/Cake/Controller/Controller.php, line 968
ExceptionRenderer::_outputMessage() - APP/Lib/cakephp/lib/Cake/Error/ExceptionRenderer.php, line 292
ExceptionRenderer::error500() - APP/Lib/cakephp/lib/Cake/Error/ExceptionRenderer.php, line 260
ExceptionRenderer::render() - APP/Lib/cakephp/lib/Cake/Error/ExceptionRenderer.php, line 190
ErrorHandler::handleException() - APP/Lib/cakephp/lib/Cake/Error/ErrorHandler.php, line 127
[main] - [internal], line ??

What should I do to fix this issue?
@adulau @iglocska Can you kindly please help me resolve these issues.

The report names error.log, debug.log, APP/View/Elements/footer.ctp line 2, and the PyMISP API path. Start by reproducing the GUI/API failure and comparing those logs; done would require a clearly identified, reproducible fix for the MySQL connection error and the undefined debugMode notice.