The problem occurs when the URL contains UTM tags and an anchored link at the end.
For example:
  1. Works (anchor at the beginning of the URL): https://site.com/some_page/#video?utm_source=sendinblue&utm_campaign=Campaign__Name&utm_medium=email
  2. Works (no anchor): https://site.com/some_page/?utm_source=sendinblue&utm_campaign=ICampaign__Name&utm_medium=email
  3. Does not work (anchor at the end of the URL): https://site.com/some_page/?utm_source=sendinblue&utm_campaign=Campaign__Name&utm_medium=email#video
Only scenario 3 gets the error: "Invalid return URL." Stripe also shows a double anchor at the end for some reason.