EAS: Calendar Entries don't sync with MS Outlook

[kashi-root]

Good morning,
i'm quite new to Group Office and i'm testing GO on a locally installed instance, just updated to 6.8.71.
I am surprised of how much activity there is in the advancement of the solution.

I did found an issue with the Calendar Sync via EAS from Outlook. If a new Entry is created in Outlook, it does not get synced. It does work with iOS and it does work the other way round. Means, calendar entries created via Webmail or iOS are shown properly in Outlook.

MS Outlook Version 2408 Build 16.0.17928 64 Bit
GO 6.8.71 on Debian Bookworm

The Z-Push Log throws the following Warnings, looks like an encoding issue:
/usr/share/groupoffice/go/core/util/StringUtil.php:472 iconv(): Wrong encoding, conversion from "pdfiso-8859-1" to "UTF-8//TRANSLIT//IGNORE" is not allowed (2)

Maybe someone already had this issue and got a solution?

Regards
Kashi