Still unresolved...

I have the same effect if I send a signed text-only or HTML email using Outlook 365 and our Exchange 365 and if I view the mail on Outlook on Android. The mail shows no contents only the file. If I view the mail using Outlook 365 on my PC or Windows 10 Mail it looks fine.
If I address it also to my Microsoft account and my Gmail account (using all adresses in the TO: field of the same mail) the email looks normal in the Gmail Android app and (!) in Outlook for Android.
So the same mail - both in the same Outlook for Android app - looks correct in my Microsoft account inbox but only shows the file in my Exchange inbox - in the same Outlook App. Weird… Nokia 7 plus, Android 9, newest patch level (September 2019) and no updates in Google Play Store.
BTW: In Exchange 365 I configured the message flow, default remote domain (there is no other) to never to use Rich Text, always and only HTML.

Hi
FYI here is what I did to resolve:
running gpg.exe and gpg-agent.exe as Administrator and XP mode....
gp-agent:
set service Priority to REALTIME
Disabled Windows UAC virtualization.

It is not just about being annoying but for security reasons. It would be too easy for other applications *think webbrowser or Acrobat) to take a screenshot and pop up a modified version of that screenshot with data entries to act as a MitM.

The message has not been encrypted to you. Ask the sender to encrypt to you.

How to fix "failed: no secret key

This is generally the better tracker to report Gpg4win / Kleopatra issues. The git systems are linked in a way that I can both automatically add a commit here and in the KDE tracker.
I just noticed the KDE report a bit quicker because there is less traffic, but I would have seen it here within the day.

Today a new signed message from BSI Buerger CERT was received. The PGP signature could be verified by first opening of the document. As I opened the file some hours later again, it failed, as I opened it a third time (shortly after the second time), the signature was verified. Outlook was not closed between the second and third opening. Signature verification appears unstable.

There is no reason for apologies :-). As far as I know this all is open source, freeware and you don't get paid for this, right? So, I simply also try to add my contribution by most precise error reports to help to find the error and am grateful if it will be solved one day in the future :-).

Gpg4win-3.1.8 was released.

As far as I know this works.

This works but might have created a regression which is tracked in T4701

I give this normal priority even if it is a whish because I have the same whish and already have some code around that would make it more comfortable, especially if it is used directly in GpgOL.

I still would like to test this some more and work on it. I think the implemnation might still be a bit fragile.

I'll try to look at it this week. Apologies for the delay with this.

Last week GpgOL again destroyed an email with a BSI newsletter - it was shown as empty after I opened it a second time - and the same is true in such cases then in Windows 10 Mail as well as using Outlook Web Access: