ned's blog
Serious Sam 2 FAQs 
Is 'Serious Sam: The Second Encounter' Serious Sam 2

No. - link -

If you are getting any type of version mismatch error then you are trying to enter a server which is a different version to your client (ie. the version of Serious Sam 2 you are playing is different to the server you are trying to connect to).

update your client to the latest version
currently it's v2.070
download - here -

If you are patched to this latest version and you are still getting this error then the SERVER isn't running the latest version.

Avoid these servers. If possible, contact the server admin and tell them of the v2.070 update.

How do I know what version of Serious Sam 2 I have?

Start the game from the desktop.
Your current version is listed near the top right corner.

What settings do you use that aren't in the menu?

hud_bShowLevelName = 1
hud_bShowPing = 1
prf_bShowFPS = 1

Screenshot - here -
These can be entered into the console as is.
See below for how to use the console.

How do I enter a password to get into a locked server?
(I was asked this often when I did some initial testing with this game)

-enter the console by pressing your tilde key (~)
-type net_strc
-press TAB, this should auto complete the cvar net_strConnectPassword
-from there type = "the-required-password"

so, it should look something like this...
net_strConnectPassword = "the-required-password"
(note: the quotation marks ("") are required)

-press ENTER

-if the password you entered is correct you should be able to enter the locked server

The above querries are only what I've received.
See below for a lot more info.

The Great Serious Sam 2 FAQ