Unable to repair boot with Windows 8 and Ubuntu 13.04

Solution 1:

See this bug report. It includes a workaround, which I quote:

1. via Gparted create another EFI partition (FAT32, 200MB, located in the first 100GB of the disk)
2. move the 'boot' flag on it
3. make Ubuntu use this new ESP (eg via Boot-Repair --> Recommended Repair).

See also the error message that you quoted in your question.

Solution 2:

In my case (using Boot-Repair from within a Ubuntu Live USB, as explained in the 2nd option of the Boot-Repair wiki), the Locked-ESP error was caused by my partitions being mounted.

I unmounted them using gParted (right-click on each partition -> unmount), and then Boot-Repair succeeded in repairing my system (using Recommended Repair).

See also this article.