Outlook & PDF's - sudden spinner of doom

This started about 3-4 weeks ago, and has now spread to about 25% of my estate.

PDF's being either attached as new or being forwarded in Outlook (Classic) leads to Outlook having spinning circles of doom on each PDF attachment that can run for 5-10 minutes before finishing.

I'd think a KB rollout, except the entire estate is up-to-date, and this is selective.

It appears to only affect Classic, not New.

I've already ran through the obvious checks and tests:

1: Cleared %temp%

2: Ran a repair on Office

2a: Stripped Office entirely, cleared out registry and file orphans, re-installed from clean

3: Ran Outlook in safe mode *ADDENDUM: This means COM's are disabled!*

4: Checked trusted settings and turned off the attachment preview function

5: Made entirely new mail profiles

6: MS have been doing routing changes, so I'd ran through a few network resets (netsh resets and a flush\\register DNS) along with trying a different DNS

Its a range of machines, specs aren't super shiny, but not garbage either - 11th gen / 12th gen i5's, all at least 16GB in dual channel if not 32GB and all on NVMe.

We're using MDO/DFE - same policies across the user portion of the estate, so again, no obvious discrepancy.

Anyone else running into anything similar at the moment and have any ideas?

**ADDENDUM:**

Also tried disabling PDF protection via Registry - no difference.

PDF software in use includes Acrobat Reader, Acrobat Pro & PDF-Xchange - but no commonality between the presence of those packages and the behavioural problems. Everyone else uses Word/Edge to open them.

**UPDATE 2-3-26:** Option 6 appears to be working after a second run of it.

No, I don't understand why!

Full sequence as follows:

0: Launch elevated CMD

1: netsh int tcp reset

2: netsh int ip reset

3: ipconfig /flushdns

4: control netconnections - set DNS to custom (I've been using OpenDNS, but that's out of personal preference and I've not been inclined to test with any others at this point)

5: restart

6: Repeat 0

7: ipconfig /registerdns

8: test Outlook - IF the issue is still presenting, repeat steps 1,2,3,7 and restart again

9: test Outlook

So far, every issue after the second restart has cleared.

Oddly, we've had one unit that had its custom DNS reset back to automatics - not clear as to why. Could have just been the analyst doing the work misclicking - can't confirm one way or the other at this point.

I am reminded of the popular IT truism about DNS. I'd wondered if there was possibly a legacy GPO trace from when the estate had an on-prem DC, but even at that, some of these endpoints are newer than the retirement of the AD - so that theory doesn't pan out either.

I'm throwing the steps into a package for Company Portal so that users can do a self-service fix and unless I start getting any more examples of the fault I'm going to call it closed and move on.