Login in not possible: $_SESSION Variable not set

Suddenly I cannot log in anymore on a lot of MODX sites: after trying to loging an error message appears:

$_SESSION Variable not set

I don't know what it means or what is wrong.

They all use Captcha. I tried to set the captcha to '0' in the database, and clear the cache through FTP, but then the entire site goes down (error 503: service temporarily not available).

What could be wrong? This is affecting many sites now.

Did you try with a different browser? Or clear browser cache?

I did. And since it's happening on a lot of sites, I'm worried.

Does this help - looks like it's still current...

https://github.com/modxcms/revolution/issues/12372

It looks the same problem.
I tried every browser, even browser with which I did not visit or login these sites before.

I emptied the cache on one of these sites, and the entire site got unavailable: error 503 Site temporarily unavailable.

This is what the error log says:

[2018-11-15 11:32:55] (ERROR in xPDOConnection::connect @ /var/www/vhosts/domainname.com/httpdocs/core/xpdo/xpdo.class.php : 3097) SQLSTATE[HY000] [1045] Access denied for user 'db25453_tmmr15'@'localhost' (using password: YES)
[2018-11-15 11:32:55] (ERROR @ /var/www/vhosts/domainname.com/httpdocs/core/xpdo/om/xpdoquery.class.php : 637) Could not construct or prepare query because it is invalid or could not connect: SELECT `modSystemSetting`.`key` AS `modSystemSetting_key`, `modSystemSetting`.`value` AS `modSystemSetting_value`, `modSystemSetting`.`xtype` AS `modSystemSetting_xtype`, `modSystemSetting`.`namespace` AS `modSystemSetting_namespace`, `modSystemSetting`.`area` AS `modSystemSetting_area`, `modSystemSetting`.`editedon` AS `modSystemSetting_editedon` FROM `modx_system_settings` AS `modSystemSetting`
[2018-11-15 11:32:55] (ERROR in xPDOConnection::connect @ /var/www/vhosts/domainname.com/httpdocs/core/xpdo/xpdo.class.php : 3097) SQLSTATE[HY000] [1045] Access denied for user 'db25453_tmmr15'@'localhost' (using password: YES)
[2018-11-15 11:32:55] (ERROR @ /var/www/vhosts/domainname.com/httpdocs/core/xpdo/om/xpdoobject.class.php : 263) Error preparing statement for query: SELECT `modSystemSetting`.`key` AS `modSystemSetting_key`, `modSystemSetting`.`value` AS `modSystemSetting_value`, `modSystemSetting`.`xtype` AS `modSystemSetting_xtype`, `modSystemSetting`.`namespace` AS `modSystemSetting_namespace`, `modSystemSetting`.`area` AS `modSystemSetting_area`, `modSystemSetting`.`editedon` AS `modSystemSetting_editedon` FROM `modx_system_settings` AS `modSystemSetting` -
[2018-11-15 11:32:55] (ERROR in xPDOConnection::connect @ /var/www/vhosts/domainname.com/httpdocs/core/xpdo/xpdo.class.php : 3097) SQLSTATE[HY000] [1045] Access denied for user 'db25453_tmmr15'@'localhost' (using password: YES)
[2018-11-15 11:32:55] (ERROR @ /var/www/vhosts/domainname.com/httpdocs/core/xpdo/om/xpdoquery.class.php : 637) Could not construct or prepare query because it is invalid or could not connect: SELECT `modSystemSetting`.`key` AS `modSystemSetting_key`, `modSystemSetting`.`value` AS `modSystemSetting_value`, `modSystemSetting`.`xtype` AS `modSystemSetting_xtype`, `modSystemSetting`.`namespace` AS `modSystemSetting_namespace`, `modSystemSetting`.`area` AS `modSystemSetting_area`, `modSystemSetting`.`editedon` AS `modSystemSetting_editedon` FROM `modx_system_settings` AS `modSystemSetting`
[2018-11-15 11:32:55] (ERROR in xPDOConnection::connect @ /var/www/vhosts/domainname.com/httpdocs/core/xpdo/xpdo.class.php : 3097) SQLSTATE[HY000] [1045] Access denied for user 'db25453_tmmr15'@'localhost' (using password: YES)
[2018-11-15 11:32:55] (ERROR @ /var/www/vhosts/domainname.com/httpdocs/core/xpdo/om/xpdoobject.class.php : 263) Error preparing statement for query: SELECT `modSystemSetting`.`key` AS `modSystemSetting_key`, `modSystemSetting`.`value` AS `modSystemSetting_value`, `modSystemSetting`.`xtype` AS `modSystemSetting_xtype`, `modSystemSetting`.`namespace` AS `modSystemSetting_namespace`, `modSystemSetting`.`area` AS `modSystemSetting_area`, `modSystemSetting`.`editedon` AS `modSystemSetting_editedon` FROM `modx_system_settings` AS `modSystemSetting` -
[2018-11-15 11:32:55] (FATAL) Could not load core MODX configuration!

What version of MODX?
Have you updated MODX lately? If so, how did you do the upgrade?

This is odd. I don't think that reported bug is causing your problem. It's about $_SESSION variables persisting too long. No one had it crash the front end.

Your initial error does suggest a problem with $_SESSION variables, but it's unlikely that that would crash the front end unless you have a snippet or plugin that uses $_SESSION variables (or your front-end requires a login).

Check the database credentials carefully in the core/config/config.inc.php file. Make sure the user listed there has full access to the DB. The error you're getting now suggests that MODX can't connect to the database using the current credentials. Have you changed the database name, password, or database user lately?

The latest version (site has been running fine for a few months on this latest version). In fact, the site ran fine for years, was always updated, etc.

The credentials are fine.

Maybe this is a different problem than the one above and it's just coincidental.