Hey @ABIP; welcome aboard and thanks for using Dropbox!
If you get the same error on other browsers as well, can you please send me a screenshot so that I can have a visual too?
On a side note, have you purchased the previously offered extended version history add-on at some point? If so, I'm afraid that you won’t be able to join a Family plan as this Help Center article outlines.
Let me know what you find!