Data decrypted, but unwanted characters in the beginning
If the data at the beginning is exactly 16 characters long every time, then it is certainly the initialization value for the CBC mode, and you can simply cut them off and output the valid data starting at byte 17.

My guess is that because that web decoder provides no specific input for the IV, it does not attempt to truncate the prefix itself, and simply outputs all the data, letting you decide what to do with it.