When you mistype your passphrase your data isn't lost, it's just not accessible.
The filesystem on the crypted device cannot be mounted, therefore no writing operations should occur on this device and your data is still intact.

You'll just need to remove the mapper device and recreate it with the ...

I use the command "/usr/bin/cryptsetup create secret /dev/loop0" to activate my passphrase protected, encrypted storage device stored in a file.
I've got the command from this howto.
It worked for a year, but now I get: "Command failed: Invalid argument".
What do I have to change?
Something with ...

At the moment, cryptsetup needs to be linked against libdevmapper.so.1.01, therefore you need to:
# echo '>=sys-fs/device-mapper-1.02.02' >> /etc/portage/package.mask && emerge device-mapper cryptsetup
Or install the latest cryptsetup snapshot, see: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug ...