Admin won't save after IP change and new SSL?

It doesn't make sense to me, but it's happened twice now. Here are the steps I went through:

1. I asked that a hosting account for a Zen Cart site be moved from a shared IP to a unique IP.
2. After the switch was made, I had a new SSL installed for that domain
3. I updated the configure.php files to reflect to use this new SSL certificate
4. The sites seemed to be fine, and were using the new secure domain on checkout

But later I found out, no changes made within the new Admin tool were being saved. For example, add a new category and hit 'save' and you just get sent back to the Admin home page. But no change or updates.

With the first site, I thought it was a fluke, but now I know it happened to another site too.

Any thoughts as to what would cause the Admin to stop saving or updating? I'm guessing some permissions got screwed up somehow? I'm just trying to troubleshoot this for future use.

FYI, one site was 1.3.8 and the other is still 1.3.7.

That's a classic symptom of incorrectly applying the June 2009 Admin Security Patch ... especially symptomatic of not correctly updating the admin version of html_output.php as explained in the instructions: http://www.zen-cart.com/forum/showthread.php?t=130161

Thank you so much. That was the one step I didn't mention in my list, but it makes sense. I thought I was careful with that update, but obviously not. But it's good to know what to do. I already feel better. Thanks Doc.