MC 1.7.10, FE Build 1156.
When worldborder is enabled, it actually doesn't do anything in game. There is no "border".
The file .json file gets created and is read at startup. /wb will show the settings correctly. There just isn't anything preventing players from crossing the defined border. I set border to 1000 and can travel right past it with no indication that I crossed the border and nothing preventing me from doing so.

Commands used:
/wb enable
/wb center here
/wb size 1000
/wb

Output:
Worldborder enabled
Worldborder center set to current location
Worldborder size set to 1000 x 1000
/wb enable|disable
/wb center here: Set worldborder center
/wb size [z]: Set worldborder size
/wb shape box|ellipse: Set worldborder center
Worldborder info:
center = [0, 80, 0]
size = 1000 x 1000
start = [-1000, 80, -1000]
end = [1000, 80, 1000]
shape = box
enabled = true

0.json file contents:
{
"enabled": true,
"center": {
"x": 0,
"y": 80,
"z": 0
},
"size": {
"x": 1000,
"y": 0,
"z": 1000
},
"shape": "BOX",
"effects": [],
"dimID": 0
}

I'm 90% sure your problem is that effects is set to [ ]

i have a diffrent problem with wb, json file is created but after server restart the original settings are back.. but it works as long as i edit it again.

@PRavenhold update to latest jenkins build

@ruphskunk I said this on IRC, but here it is again for the record - try a /wb effect add knockback 0 as a stopgap measure to prevent players from crossing the border. I'll look into stopping players from moving through a worldborder.

Should be somewhat fixed in af70577

Sorry for reopening this, but I am new to this github and I have exactly that same issue, but the "should be somewhat fixed in af70577" ... tells me absolutely nothing.
What am I supposed to do with it?
Do I need to download a never version of FE? I am running mc 1.7.10 and I have the latest FE for it...
btw. WB has not even a config present O_o ("The configuration file for this can be found in /ForgeEssentials/WorldBorder/config.cfg"- nope.. there is nothing like that in my PC)

I have also tried ... /wb effect add knockback 0
and /wb effect add knockback 10
but nothing happens... I simply walk past the border :/