I’m using GeneratePress 3.0.2 (flexbox option active) and GP Premium 1.12.2
After recent update of Yoast SEO plugin to 15.1.1 I noticed that when I go to Customize | Appearance there appears a white page with 503 Service Unavailable error:
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.
I tried deactivating all plugins and activating them separately to find out what is causing this error. I found that:
1. The error does not appear with theme TwentyTwenty
2. The errror does not appear with theme GeneratePress if I keep the plugin GP Premium deactivated
3. When I have GP Premium active then I can have either the plugin Yoast SEO 15.1.1 or Woocommerce Multilingual 4.10.3 active without any error. Activating both gives the error I mentioned above.