White Screen of Death

Errors like this are logged. Check the error log on your server. If you can’t find the log, please contact your host.

Meantime, enable wp_debug and wp_debug_log and after an error, look at wp-content/debug.log to see if anything gets logged there. https://www.ads-software.com/support/article/debugging-in-wordpress/

You can also try this: Please attempt to disable all plugins, and use one of the default (Twenty*) themes. If the problem goes away, enable them one by one to identify the source of your troubles.

If you cannot access wp-admin, there are other ways to deactivate plugins:  https://www.ads-software.com/support/article/faq-troubleshooting/#how-to-deactivate-all-plugins-when-not-able-to-access-the-administrative-menus

Thank you Steven. I will take a look at the errors and enable the debug.

We use a custom theme, could this be the issue then? If I revert to a Twenty* I would have to rebuild the website, kind of defeats the purpose of custom themes.

Change the theme for 30 seconds as a test.

Yup, it is the theme. Thank you Steven.