Ran into this issue recently, and it’s a bit of a weird one that you might possibly run into as you’re migrating to Office 365.

Here’s the scenario:

A cloud user sends a meeting invite to someone on prem – on prem user accepts the invitation, and everything’s all good (so far). Cloud user updates the meeting invitation and attaches a document, and sends the meeting update out. All of a sudden, the on prem user can no longer open the meeting, or access the attached document. The error only seems to happen between users who were already migrated when they were sending the meeting invite back to on prem users – sending meeting invites back and forth between cloud users wouldn’t reproduce any issues; on prem users could send back and forth just fine also without issues.

First thought of course, was that there was something messed up in the sharing settings as the invite crosses orgs, or possibly something broken in hybrid – wouldn’t be the first time some weird firewall rules caused issues in hybrid, right?

However, it wasn’t a hybrid issue at all, it was something even weirder – the cloud to on prem side of the scenario was actually a red herring… that’s the only way it was happening, but the issue is actually caused because all the migrated users had also been upgraded to Office Pro Plus (2016), and the on prem users were still on Office 2013!

The issue: HTML Calendaring

Turns out there’s a known issue that can happen with meeting requests that are created in Outlook 2016 and then opened in Outlook 2013. According to the support article, the error has to do with the way Outlook 2016 formats calendar requests, particularly meeting requests that have “table content, embedded images, and attachments”.

I tried to reproduce the issue in my own environment, and here’s what I found…

Starts with a regular calendar invite – everything works fine:

Then I went back and added a document, table, and image to the meeting request (might as well do them all, right?)and sent out the update:

Sure enough, opening the attachment in Outlook 2013, the HTML options (image, table, attachments) are either messed up, or missing, or corrupted. If the message appears corrupted, the recipient won’t be able to open it at all.

In my case, I was still able to open the meeting request and the attachment, but the content was definitely messed up.

Interestingly enough, the content looks fine in OWA:

The Fix:

Option 1: Upgrade to Office ProPlus

If you have the ability to upgrade your users to Office ProPlus, this is by far your best option – your users will be using the latest and greatest, and won’t run into issues like this again.

Option 2: Update Office + Registry Settings

According to the support bulletin, you need to install KB3127975, and then add these registry settings:

1. Open your registry, and navigate to ComputerHK_CURRENT_USERSoftwareMicrosoftOffice15.0OutlookOptionsCalendar, right click, and choose New – DWORD (32-bit) Value:
   

   
   

1. Name the key AllowHTMLCalendarContent, and then edit it and change the value to 1:
   

   
   

Exit the Registry editor, and restart Outlook, and you should be good to go.

I’d still suggest upgrading to Office Pro Plus if you have the licensing to support it – in the meantime, however, this will at least make sure that your users are not having issues opening HTML meeting requests. Hope it helps!