I'm running RHEL 5.10 and connecting PC running Windows 7 to it. As long as I have "server signing = disable" then it's work and when I set it to "server signing = mandatory" it's fails. Here's what I have in the file:
[global]
lock directory = /var/spool/locks
workgroup = xxxxxxxx
security = server
netbios name = xxxxxxx
server string = aicftp samba server
password server = xxxx xxxxx
bind interfaces only = False
log file = /var/log/samba/%m.log
max log size = 50
guest account = nobody
encrypt passwords = true
deadtime = 15
domain master = no
preferred master = no
# create mask = 0744
# directory mode = 0775
unix extensions = yes
restrict anonymous = 2
idmap uid = 16777216-33554431
idmap gid = 16777216-33554431
template shell = /bin/false
winbind use default domain = no
# server signing = mandatory
server signing = disabled
# client use spnego = yes
client signing = mandatory

Any assistance will be appreciated...

i spent a little time researching this because, well i was bored and wanted to. i was reading this page...

the-basics-of-smb-signing-covering-both-smb1-and-smb2

i from reading the article it states that if you set the client signing mandatory and server signing disabled, both communications will be signed and secured. it is not needed to set both to mandatory so if you have it working as client mandatory and server disabled, go with it. it also specifically states in the article that it is not a good idea to change the settings on those which are by default client mandatory and server disabled.