mobile

Hey guys!
It's been a long time since I've stopped by, but I hope you can help me :)

Last week, I decided to give Windows 10 for Phones a shot on my Lumia 820. It was a complete pain in the ... ;) Basically nothing seemed to work properly, so I decided to go back to WP8 / WP8.1.
The downgrade to WP8.0 with the Recovery Tool worked almost without any difficulties. Getting my Backup to work wasn't a problem either...
After I got the main stuff (Homescreen, App settings, etc) up and running, I installed the Preview for Developers App to upgrade to the latest version of WP8.1 to get Cortana working here in Germany :)
But after the first two updates, I still don't have Cortana - I'm stuck at WP version 8.10.14192.280.
The Cortana build should at least be 8.10.14219.341.
Every time I try to check for updates, WP comes up with Error Code 80072f8f.
After hours of searching and reading, the only cause for this error I could find was a wrong set date, time or region setting.
Date, time and region are all correct, it doesn't make any difference if i set it automatically or manually. Store apps can be installed and updated without any problems - which shouldn't be the case if any of the above settings were wrong. Updating to the phone`s current OS version worked flawlessly with those settings.
I've manually checked for updates about 30 times over the last few days, to rule out any server-side problems regarding windows update.
when the phone is connected to my pc running Windows phone recovery tool, my pc can't find any newer version of WP either.

Am I doing something monumentally stupid? Or is there any known issue I am encountering? I have no idea what to do anymore... :/

Btw. The about section in settings shows the following:

Model: Nokia Lumia 820
Carrier: o2 DE - v294.02
Software: Windows Phone 8.1 Update

OS version: 8.10.14192.280
Firmware revision number: 3051.40000.1347.1005
Hardware revision number: 2.4.5.0
Radio software version: 1.0.202158.3
Chip-SOC-Version: 8960

Thanks in advance ;)