I found the solution here: http://www.wpbeginner.com/wp-tutorials/how-to-fix-wordpress-posts-returning-404-error/
“Login to your server using FTP, and modify the .htaccess file which is located in the same location where folders like /wp-content/ and /wp-includes/ are located. The easiest thing you can do is to temporarily make the file writeable by changing the permissions to 666. Then repeat the original solution. Don’t forget to change the permissions back to 66”.
Pretty permalinks are working again and also the custom 404 page.
Thank you so much for your quick reply. Tt turned out to be the memory-limit. I changed it and it works now. Happy I am with this beautiful and outstanding plugin! Best wishes, Connie
