It's not possible to authenticate a user, who contains a '%' in his username, using smbclient, net or other client tools.

# samba-tool user setpassword testmit%prozent --newpassword=Passw0rd
# smbclient -L localhost -U testmit%prozent%Passw0rd
session setup failed: NT_STATUS_LOGON_FAILURE


Workaround: Use Kerberos ticket for authentication

# kinit testmit%prozent@MY.DOM
Password for testmit%prozent@MY.DOM:
# smbclient -L host.my.dom -k
        Sharename       Type      Comment
        ---------       ----      -------
        netlogon        Disk      
        sysvol          Disk
...