Jen

Thanks, I followed those suggestions. I only had one user and it was an administrator, but I did check (OK)/repair (nothing to repair) on the user, user table, and database. I also created a new admin user, but that had no effect on the admin pages that were resulting in 404.

I went ahead and deleted all themes except twentyfifteen again, and all plugins except akismet/hello (from fresh install) which had no effect — I still had 404 pages on themes and plugins. However, after copying my superhero theme back, I was able to use the “Customize” menu to change the theme back to superhero. (wp-admin/themes.php still results in 404, but I don’t care so much now that i have a way to access themes via customize if I need to.)

The only remaining issue, which is a bummer, is that there’s no way for me to administer plugins. The plugins page still results in a 404, even when everything besides what comes out of the 4.3.1 box has been deleted from wp-content/plugins folder.

Thanks again for your help! Even if it isn’t resolved it is very helpful giving me things to try / look at.

Thanks for your reply. I have been trying to troubleshoot. Tried Debug and just get a bunch of references to the following over and over again:
[22-Nov-2015 02:37:25] PHP Notice: The called constructor method for WP_Widget is `<strong>deprecated</strong> since version 4.3.0! Use
<pre>__construct()</pre>
instead. in /wp-includes/functions.php on line 3457. `
Is that a clue? I replaced the functions.php file with the new one from 4.3.1.

I don’t believe the theme is the issue, because the main problem is that I’m unable to use several wp-admin pages that *aren’t* using the theme (including the page where I can select the theme). Just to test that theory, I manually switched the theme by renaming the theme folder, but it had no affect on my ability to access admin pages. Also switched the theme (manually) to latest Superhero, and to one of the ones WordPress ships with. That had no impact on my ability to access the admin pages that are resulting in a 404 error.

I completely deleted and did a fresh install (from latest 4.3.1 download) of the contents of:
wp-admin
wp-includes
wp-content/plugins
… even then, I still am unable to access admin pages such as Themes, Update, or Plugins. Here are admin pages that result in a page not found error:
update-core.php
edit-comments.php
themes.php
Header
Plugins

I’m going crazy trying to figure out what the problem is here. Can anyone help or point me in the right direction?

I should also mention that I checked my WordPress database; it appears to be fine. Used host’s database check tool (all appears OK); and looked at various content pages including the “missing” one via phpmyadmin — the content is there in the database and there doesn’t seem to be any problem with it.

Hi Kathryn,
Changing the tag word totally worked – go figure! Thanks for your help.
Jen

Hi Kathryn,
Thanks for your response! I installed and tried two different plugins, the one you listed above (“Regenerate Thumbnails”) and another (“Force Regenerate Thumbnails”), and both appeared to have successfully regenerated the thumbnails for each of my three featured posts, but neither worked in terms of getting the home page slider to re-appear. I double-checked everything, including the image sizes (each featured image is 1200px wide, one is 1280). They are all tagged “featured” which is the featured content tag in my reading options. Jetpack is installed and working fine, and I also updated all of my plugins, just in case. (Also used FTP to make sure the theme files hadn’t been modified, and cleared browser caches and tried multiple browsers in case it was CSS related.) I tried de-activating and re-activating the Superhero theme, too. Shoot! Do you have any other ideas about what it might be, or what I should try?
Thanks,
Jen