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.
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:
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.