Hello, Martijntje,
Thank you for reaching out to us.
1/ Please enable debug mode in WordPress – https://wordpress.org/support/article/debugging-in-wordpress/ to find out what error appears on your site, it will point you in the right direction and what needs to be fixed.
2/ Could you please provide temporary wp-admin access? We need to check your settings.
To grant WP-Admin access, please proceed to create a new user account with an administrator role through your WordPress Dashboard. Once the account is established, you may securely transmit the username and password to us via the Private Content section designated for this purpose.
Warm Regards,
The 8Theme Team