Thank you, Andre. Just updated here, problem fixed.

Yes :-). I see it also in line rows (many '_' characters). As I wrote, I (and probably all others) always exptected this to be a formatting error on the sender's side :-).

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.

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 :-).

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:

If helpful I can demonstrate or let you debug in a TeamViewer (I have a license) or VNC remote session in a fresh VM.
For sure this is not urgent for me. So, take your time!

Hi Andre,