As per Case #680270, Virtualization Manager is unable to accept the authentication of LDAP users with long passwords/passphrases. In our case, the user had over 30 characters in their passphrase. We confirmed that changing to a short (under 15) character password worked. We did NOT test each individual iteration of length.
As passphrases start to be (marginally) more popular the use of long passphrases should be allowed for LDAP users.