Hi, thank you for writing this script!
I noticed that downloading from private groups appears to have broken for some reason. The included test files fail on _test_public_2_with_cookie() as well. The error is:
Warning: error trying read config from the 'curl-options.txt' file
:: Unable to find any mail messages from ./viettug.org-google-group-crawler-public2//mbox/
It seems like this error is being thrown by curl itself? I'll try to look into it some more but not the most familiar with these tools. I have curl 7.38.0 since I am on an old version of Debian, but it doesn't seem like either the user-agent or the header options have changed and this was working a few weeks ago.