Steps to reproduce:

1. Create new API user;
2. Add groups "Internal users", "No access to the frontend";
3. Add super admin role to this user;
4. Log out and try to sign in to the user;
5. Observe, that you receive error "No permissions for system access."
6. Open console and execute curl command:

curl -k -w "\n" -s -X POST -H 'Content-Type: application/json-rpc' -d '
{"jsonrpc":"2.0","method":"user.login",
"params": {"username":"zabbix_api","password":"yourpassword"},
"id":1,"auth":null}
' http://yourIP/master/api_jsonrpc.php 

Result:

Expected:
API successfully executed, result received.