Outlook 2019, Outlook Proplus v1808/1809 Unexpected Calendar Cache

Outlook version
Outlook 2016 64 bit
Email Account

You are just one of our favorite people! Your website is such a great Go-To for every problem with Outlook and Exchange we use it constantly.

Since late September we have been investigating a mysterious problem with Outlook. Affects 2019 Preview, and Office 365 Pro Plus versions 1808 (September 11) onward.

For MAPI/POP/PST folders, on SOME computers, creating an instance of a recurring ends up cached, and remains that way until Outlook is closed.

1. Create a recurring event in Outlook Calendar (Weekly, 3 occurrences)
2. Edit the second instance by dragging and dropping to another time.
3. Edit Primary record in the series, and choose "Entire Series".
4. Make any change. Click Save and Close.
5. Issue: Receive an error trying to save the change "The item cannot be saved because it was changed by another user or in another windows. Do you want to make a copy on the default folder for this item?".

Note: If you edit A DIFFERENT instance, make a change, cancel the edit, it prompts do you want to save, and say no. However the pattern commits and step 5 will succeed.

Note: Some PCs with the same releases work fine.
Note: Closing Outlook and reopening will clear the error.
Note: 32-bit and 64-bit both exhibit this problem.
Note: Exchange folders - no problem at all - this is standalone folders only ost and pst.
Note: ScanPST does not change the behavior. On the systems affected, this is 100% reproducible.

Any ideas here? It seems to be running afoul of some multi-user logic in the IMAP folder system that is failing to flush the instance at the end of the edit. We are quite mystified on how to work around it since it is Outlook itself that is failing here.

Thanks! - Wayland
Outlook version
Outlook 2016 32 bit
Email Account
Office 365 Exchange
This is in the "Calendar (this computer only)" folder? It sounds like its sync issue (mail will generate a similar error) - but calendar doesn't sync.

for mail, the fix is to select another item or switch folders. I'll see if I can repro it.
Outlook version
Outlook 2016 64 bit
Email Account

Our only computer that can reproduce this inhouse is running OL2019 Preview v1811

I was not able to repro on Office 365 v1808 and v1809. I switch now to OL2019 Perpetual v1809. (all IMAP folders here).

We have one confirmed customer report, and two unconfirmed reports; v1808 and higher, both 32 and 64 bit.

I'm thinking it may have to do with the origin of the PST/OST file. That is; if the file was created by some older version, it has the problem. If newer than a version, it does not have the problem. Just a guess.
Outlook version
Outlook 2016 64 bit
Email Account
I have an update. Our PC that was failing auto-updated Outlook. It is now on v1812 and no longer shows the problem. So this problem seems to be found and fixed by MIcrosoft. Consider this issue closed.