All of our login IDs are LDAP/Active Directory names. We can log in. When we upgraded the Active Directory plugin to 1.26, we noticed that the "Configure System" contained error indications. Specifically, under "Authorization", there is a "user/group" list, and each username had been replaced by an error indication. Examination showed the same error for each username. For example:

org.acegisecurity.BadCredentialsException: Failed to retrieve user
information for d_lindsay; nested exception is
javax.naming.NamingException: [LDAP: error code 1 - 00000000:
LdapErr: DSID-0C090627, comment: In order to perform this
operation a successful bind must be completed on the
connection., data 0, vece\ufffd]; remaining name
'DC=ocarina,DC=local' at
hudson.plugins.active_directory.ActiveDirectoryUnixAuthenticationProv\
ider.retrieveUser(ActiveDirectoryUnixAuthenticationProvider.java:231)

Reverting to the 1.24 plugin fixed this.