It's because not all Open Beta servers are using the same release that you have.
Players have to have the same version of ESF as the server has. (exactly same client.dll file)
Your version of Open Beta is old now, but most servers are even older.
(to make this easier, I'll copy and paste my list thing)
R1 - 5th Jan 07 --- no patch for ESF B1.2.3 -> ESF B1.3 OB-R1
R2 -20th Jan 07 --- 1st patch (R1 -> R2)
R3 - 4th Feb 07 --- 2nd patch (R2 -> R3)
R4 -18th Feb 07 --- 3rd patch (R3 -> R4)
R5 -18th Mar 07 --- 4th patch (R4 -> R5)
R6 - 9th Apr 07 --- 5th patch (R5 -> R6)
Newest version is R6.
You have R5.
The current two main dedicated Open Beta servers...
- .Maze's ESF 1.3 OB Server uses R4. (.Maze said he's changing this server to normal ESF B1.2.3)
- ESF 1.3 Testing Grounds uses R4.
If you are lucky, some n00b who happens to have an up-to-date release and doesn't know how to tick "Lan Game" might create a server named "Earths Special Forces Server".
And if you are lucky, you will be able to connect to it... but it will probably crash very quickly as it won't be a dedicated server...
I occasionally host my own laggy Open Beta server that always uses the newest release, but no-one ever joins...
I don't even know if anyone can join... might be some problem stopping them... or they just realise my server is crap before joining...
That's sadly about as good as Open Beta servers get!