It's the same thing with me. Everything was working fine then I applied the update taking me to StreamFab 64-bit 6.1.3.1. It doesn't matter if I try to downlead a music file or video file. It barely starts to download and that's when the message appears.
ChristmasRose
Sorry...I meant to add that I did Google the phrase "expected string or bytes-like object" and it did indicate it is an error that appears when using Python (whatever that is), but the solution is not something we users can do or control. I went so far as to use Revo to uninstall the product completely, then after the re-boot, I re-installed the application and re-authorized it but I have the same resulting error when trying to download anything from YouTube on channels (and videos) I've had no problems with in the past. (The reason for trying to download a video and a music file I've already downloaded was to insure it wasn't something connected to a new video/music file.)