MQTT Setup - did it change

Yes but that config is saved in the json database to which you have no direct access.
The only way to configure it is via the paperUI. (Unless you want to start fiddling with the json database manually)
So I recommend that you remove the link in the paperUI and create the link in your items file
as I have shown you above and again here:

Switch ErIsPost “Er is post” { channel="mqtt:topic:MQTTgeneric:postSchakelaar" }

I have copied your link in my items file.
Did a reset of the openHAB Raspberry

Now it is working perfect

Many thanks @vzorglub

Please tick the solution post, thanks

How do I tick the solution post?

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