First try updating the key:
Credentials > Manage A2A > Clients > <double click your client> > Change Key
If that does not work, clear the client cache file:
1) Stop the client:
$CSPM_CLIENT_HOME/cspmclient/bin/cspmclientd stop
2) Delete the following if it exists:
$CSPM_CLIENT_HOME/cspmclient/config/data/.cspmclient.dat
3) Deactivate A2A in the device: Devices > Manage Devices > <select your device> > Unselect 'Active'
4) Start the client: cspmclientd start
5) Activate A2A in the device
These steps plus some additional troubleshooting is outlined in the following tech tip:
https://communities.ca.com/docs/DOC-231173375-tech-tip-troubleshooting-a-failed-a2a-request