I'm having this problem now. I tried upgrading to the latest pre-release and it's not working.
This is the message in my log. It is similar but slightly different.
2020/12/22 01:53:19.233263 [ERR] Failed to download xmltv: xmltv unexpected ctype: application/binary
This was working for me yesterday and as far as I know there are no changes to the data. My XMLTV guide data is created daily using Webgrabplus - I have not updated the app or made any changes to it. My guide data is a file that is not zipped and is hosted on dropbox.
It was working yesterday but I noticed tonight all the guide data is blank. Let me know if there's any additional detail I can provide.
Thanks!
PS For clarity's sake, it was not working before I updated. When I found this post, I tried updating and that did not fix the issue.