When thetvdb upgraded their API they changed favorites from returning Series ID to returning Show Names. flexget thetvdb_list plugin expects an integer and gets back a string. Thus the "invalid literal for int() with base 10" error.
I posted this issue on thetvdb forum https://forums.thetvdb.com/viewtopic.php?f=122&t=60239 and its listed as something they are looking at but they summarized the change wrong so who knows.
I was not able to get v2 responses back from https://api.thetvdb.com using Accept:application/vnd.thetvdb.v$VERSION but i did find a server that responds with v2 Series ID int() https://api-dev.thetvdb.com/
If you feel comfortable editing
BASE_URL = 'https://api.thetvdb.com/'
BASE_URL = 'https://api-dev.thetvdb.com/'
You will have relief from this error at least temporarily. However, I do not know how long this server will continue to respond or what its response will be. Based on the domain name it appears to be an official thetvdb server, but i do not know. Use at own risk.