I get this same error with a PS 1.7.3.3 original on PHP 7.1.
The weird thing is when I look at the errors. The first four entries seem meaningless repeats of "object(role)" but the fifth item starts with "at UsernamePasswordToken->unserialize('a:3:{i:0;".
I guess it should contain the username with which I logged into the backoffice. However, in this case it showed the email address to login to a different shop on the same server - in which I had just logged in. Note that this is a XAMPP installation and each of the shops is in a subdirectory from localhost.
This error happened only on this common server. On the server where this shop was installed standalone (with PHP 5.6) there was no problem.