pickle.dumps() produces a bytes object. Expecting these arbitrary bytes to be valid UTF-8 text (the assumption you are making by trying to decode it to a string from UTF-8) is pretty optimistic. It’d be a coincidence if it worked!
One solution is to use the older pickling protocol that uses entirely ASCII characters. This still comes out as bytes, but since those bytes contain only ASCII code points, it can be converted to a string without stress:
pickled = str(pickle.dumps(obj, 0))
You could also use some other encoding method to encode a binary-pickled object to text, such as base64:
import codecs
pickled = codecs.encode(pickle.dumps(obj), "base64").decode()
Decoding would then be:
unpickled = pickle.loads(codecs.decode(pickled.encode(), "base64"))
Using pickle with protocol 0 seems to result in shorter strings than base64-encoding binary pickles (and abarnert’s suggestion of hex-encoding is going to be even larger than base64), but I haven’t tested it rigorously or anything. Test it with your data and see.