|This pic is always going to be useful for something...|
I ran into this error when I went to continue patching LOTRO, saying it was unable to decrypt some of the files it downloaded.
After googling about a bit one solution one I found was where the guy went into the program files, into turbine, into the lord of the rings file then used the search function (ctrl-f) and searched for files that had been changed in the last few days where it happened.
After some experimentation (and game crashes! Yikes!) I think this is the one that was the problem. Once deleted the client downloads it again.
Use at thine own risk, if that didn't already occur to ye! Just putting out a possible solution because I had real trouble hunting down this answer, so just repeating it to make it easier for latter folks to find.