Hi,
Most probably your issue is caused by some plugin i suggest you to disable manually plugins that are installed by logging to ftp and then go to wp-content/plugins/
Now you should rename the plugin folder names, rename all plugins by prefix them folder names with 1 from example, after you prefix all plugins, and your site not displaying the whitepage enable plugins one by one by removing the prefix of certain plugin. In this way you will find what causes this.
Also you can enable wp debug / https://codex.wordpress.org/WP_DEBUG
Cheers
Radu
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