[Plugin: Flattr] unable to handle IDN (international domain names)

Hi.

The flattr plugin does not work with IDNs.

running my blog on https://www.netzgespr?che.de (which will be resolved to https://www.xn--netzgesprche-ocb.de)
Whe i enter these values in general settings WordPress address (URL) and Site address (URL) the blog is working.

when trying to auth the plugin, no oauth token is generated.
Link looks like this:
https://api.flattr.com/oauth/authorize?oauth_token=&access_scope=read,readextended,click,publish
In plugin settings callback domain = https://www.xn--netzgesprche-ocb.de

when i change the URL (and all the settings in WP) to a non-IDN (www.witzman.de) everything works fine and a token is generated.

https://www.ads-software.com/extend/plugins/flattr/