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I had a look at Prestashop 1.7.7.1. There this function in AbstractToken.php looks slightly different. I implemented the new variety and the error disappeared.

So the old version is:

    public function unserialize($serialized)
    { 
        list($this->user, $this->authenticated, $this->roles, $this->attributes) = unserialize($serialized);
    }

And the new version is:

    public function unserialize($serialized)
    {
        list($this->user, $this->authenticated, $this->roles, $this->attributes) = \is_array($serialized) ? $serialized : unserialize($serialized);
    }

The full path of the file is /vendor/symfony/symfony/src/Symfony/Component/Security/Core/Authentication/Token/AbstractToken.php

It may well be that this Prestashop "fix" only hides the real problem. But at least it makes all functions in the backoffice accessible again.