Please make sure that you are signed into the Splice desktop client with an account that has a Circle 2 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.
If your plan is active and Splice app is open, please try logging out of the Splice app and log back in. Relaunch your DAW and initiate a new instance of Circle 2.
This fix may also apply to users being prompted for a serial number.
If issues persist, please contact firstname.lastname@example.org.