I am currently using the rent-to-own subscription from Splice. When I load Serum in FL Studio, it works fine. After saving a project, then loading it again, the plugin resets to default settings and asks me for a serial number. I've tried updating, reinstalling, etc.
Using FL11
Serum v1.2.1.4
Windows 10 Pro 64bit
I replied to your email, but here is the info again.
Sounds like you have 2 different versions of Serum installed.
Go too FL options->Manage plugins, look for Serum in the list, delete in Windows from every and all location mentioned there (expand plugin manager wider to the right to see 'path').
Then open the song and it should say Serum can't be found.
Then re-run the update.
Hope this helps,
Steve