Prosites Checkout Page 404 Error - conflict with Automessage Plugin

Hi,
I have a very strange issue with Pro Sites and Automessage plugin. When Automessage is activated, the Pro Sites checkout page showing 404 not found error, but after page refresh the checkout page is loading properly.

I tested with following steps:
1) Signup to page primepager.com and create new site/account (14 days trial account)
2) Go to Dashboard and click on link: "You have 14 days left in your PrimePager free trial. Checkout now to upgrade to Premium features »" (screenshot attached)
3) The 404 page appears instead of checkout page (screenshot attached)
4) After page refresh the checkout page is finally loaded

I have this problem only with Automessage plugin enabled. Can you replicate this issue?
I granted the access to the website.
Best regards,
Damian