hello
today I ran into this problem, and in the last version available today (I think it was 1.16.18_20)
when checking the system log I could see that the error is due to the file being treated as a ".tar" file without compression.
When I downloaded the file on another host, unzipped it, and used the url with the .tar file, it managed to extract and process the list of categories correctly.
Here is the log that showed what the package was trying to do:
Sep 10 14:01:33 firewall php[10138]: squidGuard_blacklist_update.sh: The command '/usr/bin/tar zxvf /tmp/squidguard_blacklist.tar -C /tmp/squidGuard/unpack' returned exit code '1', the output was 'tar: Failed to set default locale tar: Error opening archive: Unrecognized archive format'
Sep 10 14:01:43 firewall php[10138]: squidGuard_blacklist_update.sh: The command '/bin/cp -f -p /tmp/squidGuard/arcdb/blacklist.files /usr/local/etc/squidGuard' returned exit code '1', the output was 'cp: /tmp/squidGuard/arcdb/blacklist.files: No such file or directory'
greetings