auth works with cn=My Name but not with uid=myname

This is probably trivial but I can't figure it out:

my OpenLDAP entry has an attribute of cn=My Name, an attribute of uid=myname and a password.

I can successfully log in using JXplorer using

cn=My Name,ou=people,o=my company

but not using

uid=myname,ou=people,o=my company (error code 49 - Invalid Credentials)

However searching with that dn is successful and returns 1 entry, so the uid attribute is in fact there.

Please advise how I could enable the second login method which I need for exim authentication.

Thanks!