Subscribe to blog dialog box is blank - no form

I'm having issues with the subscribe to blog by email dialog box loading blank. It happens intermittently with both the subscribe icon in the toolbar and the subscribe module when it is set to dialog form.

Sometimes the dialog box displays the sign up form, but other times the dialog box opens and it's completely blank. When I check the code in the dialog box via the browser inspector, there is no content.

You can duplicate this on my website by going to another page, then returning to the blog - or reloading the blog page - and try again. Once it stops working it fails repeatedly on subsequent attempts. Here's a video illustrating the problem = https://cl.ly/3206346489d3

I've tested this with Google Chrome, Firefox and Safari browsers and they all do the same thing. I tried clearing and disabling cache, compression and combining scripts but that makes no difference. There are no PHP errors shown in the inspector console.

All software and extensions are the latest versions:
Joomla 3.8.11
EasyBlog 5.2.7
YOOtheme Venice 1.0.8 template

Hosting is SiteGround cloud server.

Thanks,
Richard