Hi @jencap82

Hmm, I just installed Extra Theme 4.9.7 on my machine and it works well with WPFTS Pro's latest version.

Could you please check the error.log on your server and find something related to this error there? Or send to me please last lines, maybe I can find the reason for the issue.

Also please try to switch ON debug mode in your wp-config.php, it may show you some message before 504 error.