Member Sign-up Error: "You don't have permission to access this page."

Does anyone know why this error shows randomly at member sign-up on my live site? I and my tester experienced this when signing up to the site. However, the error does not appear all the time. And I never seen it during my unit testing before my tester experienced it on Sunday. On Monday, it occurred again for me, but not all the time.

When that error appears - immediately after submitting info for member sign-up, I noticed that the URL that the error page has references ‘plans-pricing,’ but my site has no pricing plans (see below).

https://www.northamericatoafricadiaspora.org/plans-pricing?appSectionParams={"restrictedPageId":"mbbp4"}