Prestashop 1.7.5.2 here.
Same problem.
Here's how it happened:
Everytime I logged in to Backoffice (1st page Dashboard) a log alert Internal Server Error was generated and I got the email about it. It happened everytime I refreshed the Dashboard page. So I figured out it must be the Prestashop Metrics module.
So I went on and disabled it. By doing that I lost access to Backoffice (just get a blank page, no errors whenever I try to enter Backoffice).
Then I enabled debug mode. Now I can access Backoffice but if I try to go to the server Logs it crashes with the OP error.
Cleared caches, uninstalled Prestashop Metrics, disable all dashboard modules, regenerated .htaccess files. Problems persists. And I have no access to Backoffice if I disable debug mode.
Now what???
Edit:
maybe these help?
CRITICAL 12:10:08phpUncaught Exception: Variable "severityMessage" does not exist.
CRITICAL 12:10:08requestUncaught PHP Exception Twig\Error\RuntimeError: "Variable "severityMessage" does not exist." at /var/www/vhosts/panacea3.gr/httpdocs/eshop/src/PrestaShopBundle/Resources/views/Admin/Common/Grid/Columns/Content/severity_level.html.twig line 47