Thank you for your answer @sergior
The issue is the same as the one described here: Impossible to access to production environment / Forest Admin Rails
We are working on a fix in forest-rails, we will keep you updated when the fix will be released.
In the meantime, as you shared a screenshot with a valid token on your install (I forgot to mention to blur the content), can you change the value of the FOREST_AUTH_SECRET on your install? It will invalidate all tokens and use a different secret for signing authentication tokens. Any random string should work here.