unknown error code during application installation: ''-21''

[nottstu]

Installing Skype, Shazam and SoundHog all seems to be fine until just before the installation completes, then I get the error message:

unknown error code during application installation: ''-21''

I suppose by virtue of it being an unknown error nobody knows what it is but maybe I'm wrong?!?

It seems odd that all of these apps which won't install have a sound based aspect but other favourites like tunein radio work fine.

Any suggestions for what's going on and how I can fix it?

Phone is a Samsung Galaxy s2 with Android 4.1.2

Cheers

p.s. ooVoo installs fine and it's pretty similar to Skype...