Unfortunately, you cannot use the same public key for several Planio user accounts. Please generate a separate keypair for every Planio account you are using. You can simplify the selection of the right key by creating a ~/.ssh/config file with the following content:
Host customer1.plan.io
IdentityFile ~/.ssh/id_rsa_key_for_customer1
Host customer2.plan.io
IdentityFile ~/.ssh/id_rsa_key_for_customer2
Please replace customer1, customer2 with the subdomains for the Planio accounts you are using.
Should you still not be able to access your Git repositories, please try the ssh-add -D command. This will remove all currently loaded RSA or SDA identities from the ssh agent. Please note that you may be prompted to enter your passphrase(s) again.