I am wondering if anyone has reported problems using HashCash plugin with iPads or mobile devices? I find no issues with a desktop view on the site using it, but repeated reports from customers they cannot use the contact form so I checked with iPad and sure enough after clicking send an error is returned that 'Security Validation failed. You may have waited too long to submit the form."
This is on a joomla 3.7.2 installation using default contact form where I have used HashCash for over a year now. I clicked send immediately after filling out the fields so waiting too long couldn't have been a real issue.