sothink crash only on encrypted swf's?

Sometimes when I try to a swf or a single script of a swf my sothink crash, searching I found that this could be due to encryption of some part of the swf, after used the "decryptor" a prg found here on the forum this don't happen anymore.
My question is: when sothink crash is ALWAYS due to some encrypted files? why the prg crash instead of give an advice that it isn't able to open the file cause is encypted?

I have that same issue, and only with decrypted files that have site protection.

And which is this decrypter that worked for you? I have tried many, and none have worked for me.