I have tried decoding your base64 encoded string and it yielded the same result. I tried encoding it again, slashes and all, and it yielded the same encoded string.
The problem is not that you are somehow getting unknown slashes out of the decoding process, but that the slashes exist there before you ever encoded the string.
The same string without slashes is this:
PD9waHAgZWNobyAianVzdCB0ZXN0IjsNCmVjaG8gJ29yIGp1c3QgdGVzdCc7ID8+
Try decoding it.
I believe that the real solution to your problem can be found here: