Red X on slide thumbnails in Master View

"Looks like MS hasn't yet fixed the problem......"

Or it hasn't fixed it for the edition of Office that you have.

Again, it may depend on which specific version you have.  Specifically, if you have one of the versions that installs itself over the internet, it can be set to auto-update, and MS releases updates quite often.

If you have the version that installs from a CD or an MSI file, there will be much less frequent updates and fixes for specific problems like this will generally have to wait until the next hotfix or service pack.

Start PowerPoint, go to File | Account.

If it says "Product Activated" on the right and there's only an "About PowerPoint" button, you have an MSI version, which might explain why no update's available.

Otherwise, it'll say something about "Subscription version" and there'll be an additional button with update options.

It's actually part of Office 365, so is always up-to-date.

Not sure what else I can do and it's causing me huge problems.

I went into a client presentation the other day, having spent weeks pulling together the content. Ended up looking like a fool, as all of my most recent changes had red crossesd across the slides.

I had to run with an older version that was incomplete; made me look unprofessional and disorganised.

What version of Office should not matter.  It is Microsoft Office plain and simple and there is a major problem with PowerPoint which has been going on a LONG time.  I too have had issues similar to DGSmith51 and lost hours of productivity.  Because of this issue I have had to change the way I work causing me to become less productive.

I looked at my version of PowerPoint and it is 15.0.4615.1000. But when I download the update for the KB you reference I receive the message that the update is already installed.  So it looks like there might be a problem with MS version numbers or something else is going on.

It is perfectly normal, as according to TechNet http://blogs.technet.com/b/office\_sustained\_engineering/archive/2014/05/13/may-2014-office-update-release.aspx KB2878315 patch was included in May update, so if you have C2R installation it is included in 15.0.4615.1001 update. This means that 15.0.4615.1000 build of PowerPoint has this fix included.

But sadly it seems that it does't help in many cases. Does anyone can tell if June (15.0.4623.1003) or July (15.0.4631.1002) updates changed anything?

So, I had this same problem. I discovered what seems to be a workaround.

I am not sure if this will help anyone, or if someone has already found this solution. It all seemed to stem from using my company's custom theme. 

To fix the problem, I changed the theme for all slides to a stock Microsoft PowerPoint theme. After all the slides reset, I then clicked back to my company's theme. I am not sure if this has permanently solved my problem, but I hope it might work for someone else with this problem.