SSH-RSA key rejected with message "no mutual signature algorithm"

Any system, running OpenSSH 8.8 or newer disables RSA signatures using the SHA-1 hash algorithm by default. When you will try to use the SSH key generated using the ssh-rsa sha-1 hash algorithm, the SSH key isn't accepted (the user receives a 'Permission denied' message) or the following message in ssh verbose mode (ssh -v):

debug1: send_pubkey_test: no mutual signature algorithm

Considering that the Virtuozzo Application Management implemented support of ECDSA keys in a 8.0-2 Platform version, it can affect the connection via SSH gate on a previous one and there is a workaround, that can be used for it:

In order to re-enable the support of RSA keys, you can modify your local ssh client configuration file ~/.ssh/config (create this file, if it does not exist)

Host GATE
   IdentityFile ~/.ssh/rsa
   HostKeyAlgorithms +ssh-rsa
   PubkeyAcceptedAlgorithms +ssh-rsa

where

         "GATE" - should be replaced with a gate hostname, you can find it in the User settings, as on a screenshot:

           IdentityFile path ~/.ssh/rsa - should be replaced with a path to your private RSA key, used for SSH connection, e.g. ~/path/to/your/key.

Or you can simply put this single line to the ~/.ssh/config, it will allow using RSA keys for all SSH connections (less recommended):

PubkeyAcceptedKeyTypes +ssh-rsa