Please follow these instructions:
1. Read following article:
https://webnus.ticksy.com/article/4415/
2. If step 1 did not help you then delete unnecessary plugins via host or wp-content/plugins and then try again.
3. Finally, if none of the above steps worked then use the following instruction to make your "WP_DEBUG" true and send your WordPress admin user/pass and FTP login information in a private ticket for deeper examination: