If some users are unable to login into Server Automation then they might have an invalid signature stored in the database.
There is a tool named verify_user_sigs that is shipped with every hotfix rollup which can be used to verify the signatures.
It is located under /opt/opsware/support/bin/ on the SA core/slice.
For details on its usage run it with the "-h" option as below:
verify_user_sigs - Verifies <SIGNATURE> field of "AAA.AAA_USER" records. If
an invalid signature is found, the user is asked if they would like to
Usage: verify_user_sigs [-y] [-n] [-ce <char_encoding>] [<username_glob>|<user_id>]
Will not attempt to fix any invalid signatures.
Fix invalid signatures without asking for permission.
Specify the encoding scheme to use for representing a string of characters
as a string of bytes. The default character encoding is 'ANSI_X3.4-1968'.
Optional user id or username glob.