Maybe it’s not really related but I was getting the same error and for me it turned out to be due to some usernames having mixed case in AD so the solution in this thread fixed it Forcing lowercase username after login - #3 by ssivy .
This was only happening when using DEX with LDAP/AD instead of the apache basic auth. Somehow the basic auth was either ignoring case or forcing everything lowercase whereas DEX doesn’t seem to have any option to do this.
Cheers,
Chris