"NO_PUBKEY C95104E509BAC46D NO_PUBKEY FB510D557CC3E840":
I tried your solution:
Do this:
1. Remove the repository.
2. sudo apt update
3. Add the repository.
4. sudo apt update
5. Install suldr-keyring.
6. sudo apt update
suldr.gpg is no longer the active key. This should be a one-time error that is cleared after suldr-keyring is installed.
-- But with no success :-(((
I found an other way to solve the problem:
1. Download the deb-keyring-file "suldr-keyring_1_all.deb".
2. Extract the keyfile "suldr-keyring.gpg" from the downloaded deb-file (it's an archive file) into a folder.
3. If not installed, install "gui-apt-key": sudo apt-get install gui-apt-key
4. Start "gui-apt-key": sudo gui-apt-key
5. Import the extracted keyfile with the started program "gul-apt-key" (import file) from the folder.
6. Renew and check the package list: sudo apt-get update
(it should work)
Best regards
Bernhard