I am seeing the same error in an app, where I embed a PDF as a base64 encoded string in an object element. Is there any way to work around this (introduced issue) for this case? 
import base64
import requests
from dash import Dash, html
# Get a sample PDF file.
r = requests.get("https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf")
bts = r.content
# Encode PDF file as base64 string.
encoded_string = base64.b64encode(bts).decode("ascii")
data= f"data:application/pdf;base64,{encoded_string}"
# Make a small example app.
app = Dash()
app.layout = html.ObjectEl(id="embedded-pdf", data=data, width="100%", height="100%", type='application/pdf')
if __name__ == '__main__':
app.run_server()