I recently went through this problem, tried all the solutions proposed here (and the internet) without success for 5 days.
In my case my environment:
Laravel: 5.5
PHP: 7.2
SSL: production
Apache
CENTOS
The problem is that I had automated the deployment using the git --bare repository with ansimble.
And all folders when pushing were with permission 0775 (inherited from the git user). When ansinble was run it replicated this permission to all folders. When composing install for example, all vendor folders also had this permission.
The csrf, has a policy of blocking what is considered insecure, especially if you use an encrypted environment (SSL).
I only realized the problem when I decided to carry out the deployment manually, zipped the project, uploaded it, unzipped it and ran the commands to generate the caches and dependencies. And then I realized that this way all folders were with permission 0755 (inherited from the system user). And this is the default permission that is considered safe.