I have F-Secure key subscription from Apple App Store. I cannot connect a device, a windows computer.
I get the message, "incorrect syncronization code".
There was a discussion: https://community.f-secure.com/en/discussion/123568/sync-issues
In addition to the opinions, points and situations described there - maybe the F-Secure KEY servers are already partially unavailable. This can be intentional, or it can be wrong.
Maybe you can to contact their Support (for example, web-chat) and ask to assistance: Contact F-Secure support | F-Secure (f-secure.com)
But good to doublecheck that all is right with devices - both solutions are F-Secure KEY; both devices are not about any other software that could interfere with connection process;
Also, good to know a bit more about your Windows device (Windows 10?); and possibility to sync / connect any device (from iOS device).