Please make sure that you are signed into the Splice desktop client with an account that has a Serum plan active before you open up your DAW to load Serum. While you are on the Rent-to-Own plan, the client needs to verify the Serum plugin once every few days.
This fix may also apply to users being prompted for a serial number. Please see below for screenshots.
If you've fully paid off your Serum Rent-to-Own subscription and are being prompted for a serial number, your serial number will be shown here: https://splice.com/plugins/plans/serum
If issues persist, please contact firstname.lastname@example.org.