same Problem on Nexus S with Whispercore 0.5.
Whispercore is a popular security custom ROM and its really a shame, that I can't use one of the best security apps (boxcryptor) on this device.
I have serveral apps that need a PRO installation from the market, too.
One of this is Titanium Backup and it works fine on all custom apps.
I really love boxcryptor and the I don't want to miss Whispercore.
So please support BoxCryptor Unlimited Key for custom roms! ;-)