Now I am not sure if it comes from there or here. I seem to have had the problem both ways:

<iframe src="https://app.acuityscheduling.com/schedule.php?owner=11392149&appointmentType=324423" width="100%" height="800" frameBorder="0"></iframe>
<script src="https://d3gxy7nm8y4yjr.cloudfront.net/js/embed.js" type="text/javascript"></script>

@drherz I can't relate that error with the code you included in the comments.

What type of post are you using? Are you able to publish the post if you get rid of the link you mentioned in your first comment?

What method did you use to upgrade? FTP? Auto-upgrade?

I did use FTP to upgrade.

Visiting the site again, http://sitesthatwin(dot)com/blog/STW.php, I notice it happens whenever I press on the headline of a blog post to go to that posts individual page.

It happens on my other blog as well, http://coach.theherzes.com/

My slugs don't seem to work either. Thus http://coach.theherzes.com/having-a-purpose takes me to the error message rather than the individual post it should.

Many FTP clients are very unreliable.

Everything here looks like some files (at least 1) have not been properly upgraded through FTP.

You should re-upload all files again AND very important verify that your FTP client doesn't report any upload errors.

Thanks, I'll give it a shot. I use FileZilla, and it's never given me a problem before.

In FileZilla, if you don't look explicitly for transfer errors, it will not warn you.

Here's the code from lines 104 through 110 in the file that Failure Notice seems to cite:

if( ! empty( $MainList ) && $MainList->single_post &&
$single_Item = & mainlist_get_item() )
{ // If we are currently viewing a single post
// We assume the current user will have read the entire post and all its current comments:
$single_Item->update_read_timestamps( true, true );
// Restart the items list:
$MainList->restart();

I went back just now, logged in again, and it works on my STW blog.

Works on the other one too. I don't know why it didn't before.

I know why it didn't before. Your files were not properly uploaded.

(and then maybe you had the error in the page cache for 10 minutes after your last successful upload).