I had exactly the same problem. When trying to change Permalink from default to any other type. Howeevr, I noticed even though you get 500 error on web page, the table “WP_options” in MYSQL would still be updated with “permalink_structure” to the new permalink type, such as “/%postname%/”. I think the problem is translation layer failed to properly handle those special characters “/%” when retrieving the value of permalink_structure from the table.
