Hi,
Could you try checking if any ad blockers are enabled since they might interfere with Klaviyo form functionality. 
Try further clear your browser cache and cookies to see if that helps. 
Also check the form settings to make sure it’s set to display on both Desktop and Mobile usage under Behaviors

Hi Fario - 

Checked for ad blockers and cleared my cache and cookies. Klaviyo is showing the sign up form should be visible on both desktops and laptops. 

Mine’s doing the exact same thing for a client; the emails are actually being submitted, but not signed up to email marketing. The message reads ‘An error occurred when submitting. Please try again later.’ and I’ve done everything instructed on various forums but to no avail.