Got 404 on login attempt (Template not found: actions/users/login) Can't login in my staging env since last full plugins/craft update (still able to in my local env).

I already...


Remove my composer.lock and /vendor files and re-install all components
Rename/delete the /web/cpresources directory
Flush all caches throught CLI command
Try to login in Private mode and other browsers


And still, I got this as a response :

{"error":"Template not found: 403.shtml"}


Here's the trace

https://pastebin.com/63NA1XQy

Here's the composer.json

{
    "require": {
        "craftcms/cms": "3.1.31",
        "vlucas/phpdotenv": "^2.4.0",
        "spicyweb/craft-neo": "2.3.5",
        "anubarak/craft-relabel": "1.3.1",
        "ether/seo": "3.5.4",
        "verbb/super-table": "2.1.20",
        "verbb/expanded-singles": "1.0.7",
        "verbb/field-manager": "2.1.0",
        "craftcms/redactor": "2.3.3.2",
        "verbb/navigation": "1.1.12",
        "pennebaker/craft-architect": "2.3.1",
        "marionnewlevant/twig-perversion": "2.1.0",
        "clubstudioltd/craft-inline-svg": "v0.1.1",
        "marionnewlevant/snitch": "3.0.0",
        "selvinortiz/patrol": "3.1.3",
        "fruitstudios/linkit": "1.1.11",
        "craftcms/contact-form": "2.2.5",
        "tomodomo/twig-pluralize-extension": "^2.0",
        "verbb/icon-picker": "^1.0",
        "craftcms/element-api": "^2.5"
    },
    "config": {
        "optimize-autoloader": true,
        "platform": {
            "php": "7.2.10"
        }
    },
    "autoload": {
        "psr-4": {
            "modules\\businesslogic\\": "modules/businesslogic/src/"
        }
      },
    "scripts": {
        "post-root-package-install": [
            "@php -r \"file_exists('.env') || copy('.env.example', '.env');\""
        ]
    }
}



–

EDIT: Apparently this issue happens on all our Craft sites (both 2 and 3) on our server, will update as soon as I found out more
Never too late to answer (but fixed weeks ago), apparently our host performed an update on the server that activated several new mod_security rules. One of them was affecting the login.