An archived album cannot be modified in any way. If you want to reactivate an album that you've archived, here's how.
To reactivate an archived album, locate it by clicking on the Filter button in the Albums section in FotoWeb Pro. Choose the filter My archived albums. Then click on the archived album you want to reactivate to open it on a separate tab, and finally click on the storage box icon twice to unarchive it.
This will make the archive active again so that you can add and remove content to it.