Seems like the same issue I had about 6 months ago and never got a response to:
https://discuss.freedombox.org/t/internal-error-500-at-plinth-main-start-page/395
I wiped the whole thing and started from scratch, and that worked for a while, but eventually the problem returned. Then I didn’t turn my FreedomBox device on for a few months, but when I did just last week, it magically worked again.