I also tried to change the Contact Item state using 2 Label-Cards with action Command, 1st with CLOSED and 2nd OPEN actionCommand.
However I cannot change the state
And to elaborate a bit more, if you need to change these Items from a UI, you really should be using a Switch. Contacts are for binary sensors. A sensor doesn’t issue command nor does a sensor receive commands. That’s what Switches are for.
The UIs are all designed for controlling and viewing your home automation. Thus it only supports sending commands to Items, not updates. Consequently, you cannot change the state of a Contact through a UI.
The URLs you are trying to use to command this Contact are web UI URLs. This is an unofficial way to command Items through web calls and is not officially supported. However, openHAB does provide q very robust REST API that can be used to command and update Items through HTTP calls. But you have to use PUT and POST type calls, not GET type calls (e.g. the type of command that happens when you put a URL in your browser). Under Developer Tools there is a link to interactive docs that explain how to craft the calls to interact with OH through the REST API.