ipv6 routing issue with ISP Deutsche Glasfaser

Hello,

I'm facing an issue with gnutls, my ISP, ipv6 and one of my mail
service provider. I initally opened an issue report with Evolution as I
noticed it when using Evolution. There's some debuggin information
available there: https://gitlab.gnome.org/GNOME/evolution/-/issues/3274

The issue is as follows:
With ipv6 turned on gnutls is very slow connecting to mx.freenet.de
when conneced at home with my ISP "Deutsche Glasfaser". Disabling ipv6
solves the issue as well as connecting to the internet from e.g. a
hotel.
I had no issues at all with Thunderbird when connecting from home with
ipv6 enabled.

Thanks,
Frank