I’ve spend multiple hours to solve thus issue.
Tried to install FLATHUB resp., using Flatpak [see step no. 3 here]
STEP 3: flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
Tried everything I could think of; installing manually, reinstall gnome, reinstall flatpak, searching forums, even install curl to see the connection, tried switch connection from ethernet to wifi, Reinstalled my router, disabled the firewall. But every time when i executed step 3 I would get: “…Unacceptable TLS certificate”
SOLUTION: REINSTALL CA-CERTIFICATE
sudo apt install --reinstall ca-certificates
So frickin easy.
I really hope this solution help someone like me out. //CALLE