[Typo3] Mail templates incorrect encoding

[Robert Markula]

Some extensions (like tt_products or sr_sendcard) use templates for 
creating plaintext notification e-mails. Problem is, all dynamic text 
that is inserted via markers gets correctly converted to unicode, but 
the static text from the template doesn't.
Let me explain. My tt_products template looks like this:

...
Überprüfungscode: ###ORDER_UID###
...

But the mail the customer gets looks like this:

...
?berpr?fungscode: ÖE-47484874
...

This is my TS for the mail encoding (forceCharset is enabled):
# Setting the encoding for outgoing e-mails
config.notification_email_encoding = base64
config.notification_email_charset = UTF-8
config.tx_kbmailcs_formMailCharset = UTF-8

What can I do?
(I'm using Typo3 3.8 with forceCharset enabled.)

Regards,
Ro