Your USB eLicenser is likely dying. This is a common issue with the first very old eLicenser model.
You’ll have to buy a new USB eLicenser and try to move your Cubase license to this new key.If this doesn’t work, you’ll have to contact support.
If you have a newer computer you can use, I would recommend upgrading Cubase Artist 9.5 which is the modern equivalent to Cubase SL. You can move your Cubase SL projects to another computer by using a flash drive and finish them in Cubase Artist. The workflow is MUCH faster and more intuitive.
If you’re on Windows, you can even use the same effects if you download Cubase Artist 8.5 and download the classic effects from Steinberg’s FTP.
ftp://ftp.steinberg.net/Download/Legacy_Additional_Content/Additional%20Cubase%20SX%20Plugins/