PBIRS / SSRS 2017 - EMail subscriptions fail when Subject or Body contain an Umlaut (äöü)

Hi,

i´m moving from SSRS 2008 (Sharepoint integrated mode) to PBIRS. When i create an EMail subscription on a "paginated report" on the new server (PBIRS), everything is successfull as long as the subject or body of the mail does not contain an german "Umlaut" (äöü). As soon as one of them is there, the PBIRS says that the message has been sent successfully but i never receive the message.

Comparing to the old server, i found that the old one prefixes the subject / body with encoding information, something like 

Subject: =?utf-8?Q?Test_Date_Param.rdl_wurde_um_26.?=
=?utf-8?Q?08.2018_11:23:02_ausgef=C3=BChrt?=

The new one / PBIRS doesn`t add this prefix, and i cannot find where to configure, also not when comparing the config files with the old server (web.config, rsreportserver.config etc):

Subject: Der Test Date Param-Bericht wurde zum Zeitpunkt "24.08.2018
10:51:57" ausgefuehrt. (<-- i changed the ü to ue manually here to get through the email :-) )

So, how could this be resolved? Can i configure something so that the utf-8 - prefix is there? The behaviour of the PBIRS in this case is really bad luck, cause in german language the default subject of a message CONTAINS an umlaut (ausgeführt / executed in english), so no messages get sent by default :-(

Thank you in advance!