Cant install with Java
when I download the client from the website, I get a ZIP folder with the extension .jar.
Neither I can open this like other clients nor with the function "Open with...". There is simply no Java there.
I have already reinstalled Java once (the latest version) as well as restarted the PC.
This is totally weird, anyone know why?
You can see that it's a jar from the screenshot you sent.
If it's not an option then it was not correctly or fully installed to your computer. Download from here for windows: https://javadl.oracle.com/webapps/download/AutoDL?BundleId=246474_2dee051a5d0647d5be72a7c0abff270e, follow the instructions until the installer is done, and then restart the PC.
You can also attempt to run Java through CMD. Open command prompt and then use the command
java -jarfollowed by the file (drag the file in).