First, when you give the "example.com/slave1", this is a subdirectory and not a subdomain definition.
A subdomain is something like "slave1.example.com".
Did you applied the fix documented concerning the sessions and line 530.
Also ensure that when you manage the master website that defined the slave site that you did used something like '
www.master.example.com" but have used "master.example.com" to avoid that access is restricted to "*.master.example.com".
If the master is not part of the websites that must be shared then you have to verify that all the website that you want to share use the same "root" definition.