What happened to the ability of Sophos Secure Mail to display plain text messages?

Since a few updates my Secure EMail (IOS 14.7, app 8.36.3, managed through Sophos Central without an EAS proxy to an Exchange 2016 server) is not able to process plain text messages any more. I first thought it will be only a temporary bug with a specific IOS and/or Secure Mail version but it does not work for at least 2-3 months now and in that time there were serveral IOS and app updates.

To be exact: the preview feature is still able to show the body of the mail (up to 5 lines) but when I try to open the full mail a message is shown that I have to open it on my PC. In german the message is: "Diese E-Mail enthält möglicherweise umgebungsabhängige Zeichen. Bitte öffnen Sie diese E-Mail auf Ihrem PC." / google-translated: "This email may contain environmental characters. Please open this email on your PC."

Since most of the plain text mails are notifications from our monitoring system it is very annoying, not being able to review the mails on the phone when I am out of office.

Update: the mails do not contain any special german characters.

added update
[bearbeitet von: kerobra um 1:42 PM (GMT -7) am 22 Jul 2021]