EDIT: please try again with v4.0.80?

EDIT: are you seeing the error after the image picker comes up, you see your boards, and you choose Save to pick one? Do you ever share your computer with another Pinterest user?

I'm testing on your exact setup -- had to restart to apply v66.0.5 -- and I can't reproduce what's happening for you. How are you signing in to Pinterest, please? (Facebook, e-mail, Google, something else?)

If you're switching accounts in one tab after you bring up the image picker in another tab, you're going to see that something's-not-quite-right problem; are you doing that? If not, sorry if these are obvious questions, but: are you sure you're signed in to Pinterest? Are you using account switching?

Would you mind clearing all cookies and trying again with a fresh login to Pinterest?