Updating the eLicenser on all systems that have older Cubase installs might work. The older eLicense app probably doesn’t recognize the new 7.5 license on your dongle and therefore it might not know that it’s valid for older versions of Cubase. In general, new licenses require the latest eLicenser app to work with it.
I think I had the same issue years ago when I installed C5 on my new system and had to test something on my C4 install on my old system. It wouldn’t work until I updated the C4 eLicense app to the latest version.