This happened to me today, and it’s very annoying. I have paid for my license, and I shouldn’t have to constantly manage it as I have had to since the license engine seem to throw a fit ever two months or so.
That said, here’s what I did to fix it.
When Scrivener 1.9.16 in my case gave the dialog that my software had been reverted to trial, I entered in the serial number I got when I purchased the program, and it activated again. The serial number is included in the receipt you got when you purchased the program.