Thanks for the screenshot! It’s quite odd to hear that this issue is only happening to some customers. Have you noticed any commonalities between customers who are having the same problem? For example, are they all using the same browser?
I took a look through the Shopify Community Forums to see if any other merchants experiencing similar issues have found a solution. Based on the responses in this post, it sounds like unchecking the Enable Google reCAPTCHA on contact forms option under Online store > Preferences has worked for others. I’m curious to know if this solution will also work for you, so I recommend giving it a try!
This very well may be a theme-related issue. Instances like these can be caused by an error in your theme’s code, and if this is the case, it’s likely an issue with your customers/activate_account.liquid file. Which theme are you using? Is it a Shopify-developed theme, or a third-party theme? If this is a free Shopify theme, I’d be happy to have our Themes team take a look into this.
Looking forward to your update!