something is confusing me;
i red that you need only one L-ASA5510-SEC-PL for setting up a Active/Standby Failover.
I installed the license on the 1st ASA and tried to setup the failover via the ASDM wizard.
It always fails, because the 2nd device can't have a 'base' license.
So does this mean, i really need another license?
Thanks for the conclusion.
Yes you would need another security plus license for the standby unit, just to enable failover license on it. The rest of the features on the ASA can be different on the two devices, as in number of contexts, VPN peers, annyconnect mobile. These things can be different on the two boxes. And this features is only available on post 8.3 versions.
In pre 8.3, you needed to have the exact same license and features on the two ASA's, which is not the case in your issue. Licensing can be a bit difficult to understand but you can get all the information regarding your setup in this doc:
Hope that helps.