Safari and other programmes do not connect to the internet, update servers etc.

The problem consists in the following things:

• Safari cannot load websites
• MS Office programmes do not connect to the internet (cannot update, cannot use OneDrive)
• No access to the AppStore, MacOS Update servers (new updates cannot be found)

These problems appear in my WIFI network and other (mostly private) networks, however the problem disappears when I start using the University VPN (Open VPN). When connecting to the VPN, the programmes can access the internet flawlessly.

What I already tried to solve the problem:

• Update MacOS to Big Sur 11.2.3
• Uninstall basically every third party programme besides Opera Browser, MS Office, Open VPN
• Erase Caches (Deleting files under "~/library/caches" in the Finder)
• Reset PRAM and SMC