Actually, these are new windows. As far as I am aware really only reed switches have been installed and the wires terminate in my cellar. I just had them installed without installing an alarm system yet.
to close the subject. I have now shot a NodeMCU from Ebay and connected my window sensors via an MCP23017 (Port Extender) via I2C.
The NodeMCU then simply sends a message via MQTT whether the contact is conducting or not.
In case someone has a similar problem, please just let me know, then I will gladly send the source code and hints for the cabling.