For me decrypting is harder so... The Description has been updated and I think it is much clearer now. Have a look and mark your question solved if it's ok.

Much better! Thanks a lot!

The description has been rewritten and the examples have been replaced to be clearer. If you don't have any questions, please, close the issue.

Say you've got the string "abcdefgh".

Take the even-numbered characters (2, 4, 6, 8): "bdfh"
Take the characters that are left over: "aceg"
Put them together: "bdfhaceg"

I have no idea why the counter showed up, but I redid it without presenting the solution in alphabetic order and it worked. Instructions could be clearer!

I think it's bugged in Python...?
I passed the first tests but then it starts demanding the answer is "Counter({the actual answer})" instead of just {answer} as described before.

Thanks for the reply, Chrono! I noticed it a few minutes after posting this, but had already given up the kata.

No, your code is different, you should rearrange your code to calculate the error count before using it to build the string.

I did the exact same thing and it didn't work for me :( I was starting to get crazy trying to figure out why.