MAke sure you remove it from paper ui. Remove all tradfri config files.
Then clear cache
Then try again. I had an issue at one point and realized in my frustration i had duplicated something. I had to restart. I manually removed from the json dB cause the paper ui did not appear to actually be removing it.
I’m sorry to say that snapshot #1655, which I think for most purposes is equal to the new M2 release, still often requires a bundle restart of the Trådfri binding in order to get Gateway and things online.
@OMR remind me what hardware are you on?
Also are you tied wired or wirelessly to your network from openHAB?
Curious why we have such a mixed bag of results.
OH is running on this HW.
OS is Ubuntu Server 16.04.6 LTS (x86_64), java 1.8.0.201-b09
My network is all wired, Ikea Gateway connected (with non-original 0.5m patch cable) to a Giga-Eth port in the back of my Asus RT-AC87U router (in the cellar).
This router is connected through a 20-30m patch cable to an Asus RT-AC66U running in AP mode. The VivoPC is connected straight to this AP with a 0.5m patch cable. This is the optimal place for my Z-Wave & Zigbee USB sticks. Also separates the Ikea GW and the Conbee for minimal interference.
Whenever the Ikea GW does not connect in OH, ICMP ping works fine.
Sometimes after a VivoPC/OH restart, the Ikea Gateway does connect, but most often not.
Usually when it doesn’t, one restart of the bundle in Karaf is enough, but sometimes I need to do it 5-6 times.
Me too.
Thanks for answer and description.
I am in process of switching to deconz, not because I have issues with tradfri beyond one light that intermittently connects. I want to enhance the features of remotes, both my Phillips and ikea remotes.
My gateway has 1.8.26
I have upgraded from 2.4.0. to 2.5.0.M2.
In 2.4.0 I had a couple of addon files in order get it working:
I left this configuration after during and after the upgrade.
The result was that for each OH restart I needed to go to paper-UI and re-enter the security code.
I removed now the addon files and enabled the 2.5.0.M2 TF binding.
Having now exactly the same behavior. Even worse.
Sometimes when entering the security key, the GW goes online and then immediately offline.
I will step back to addon files from 2.4.0.
Just FYI:
I asked lots of questions to Ikea support, one of them was this:
Answer from Ikea:
This could be mDNS issue which we are working on. Stability fixes are coming in the next Gateway update that is released next week. Additional fixes will come later this fall.
Interesting! Good info!
After gateway firmware 1.9.27 update it has worked 100% Never seen it offline after that.
(on M3)
Mine is still broken 
I have to reboot the gateway to get it back online, however it does come back online in openhab without me doing anything.
I’ve had this issue for months (april?), but no time to do any troubleshooting. Is it possible to only update the binding to solve this? I’d like to run 2.4 until 2.5 is released…
I’ll answer my own question in case someone else has the same issue. My Tradfri gateway came online after updating to the latest binding.
Following the guide here
http://blog.haupt.xyz/index.php/openHABconsole
using this address to get the binding
https://openhab.jfrog.io/openhab/online-repo-snapshot/2.5/org/openhab/binding/org.openhab.binding.tradfri/2.5.0-SNAPSHOT/org.openhab.binding.tradfri-2.5.0-SNAPSHOT.jar
On M4 it started acting up again 
For me the gateway only came online when I disconnected the gateway from the Ikea Home Smart App.
Took me a long time to figure this out, hadn’t read it on any forum yet.
I can’t get this to work anymore. Thing always says “Status: OFFLINE - COMMUNICATION_ERROR Error connecting to gateway.”
I already removed the thing (paperui), clean-cache, restart, reboot, gateway reset to factory defaults, removed/reinstalled binding, nothing helps
I’m on OH2.5, gateway is 1.9.27
Any idea?
Never mind guys, problem solved
May I ask how you solved it?
Thanks, Dietmar
PEBCAK issue 
I was using the serial number instead of the security code, took me 12h to discover…
Well,
seems it not solved completely afterall.
Every couple hours my gateway goes offline and can not even connect anymore with the ikea app.
All I need to do is power cycle the gateway, restart the binding and then it works for a couple hours.
Do I have a malfunctioning gateway?
Greetz,
Eric