User Sync Plugin causing 502 error

When I activate the User Sync Plugin and try to sync all the users, I'm getting a 502 error.

Also, I'm having user sign-in issues.

When I sign in as a user on the main site, I can access other sites, but when I sign in via the other child sites, I'm not logged into the main site. I was hoping the User Sync plugin would resolve this issue.