Here’s a topic with some pointers, for what it’s worth …
As for the initial error message, the forum search has this …
A hit in the Beta category says a Fairphone OS beta tester ran into this error message a while ago with fastboot versions 30.0.4 and 30.0.5, in the end 30.0.3 worked fine.
(Access to the Beta category is restricted, so no link or preview.)