This is a recurring issue with the CDN77 - something goes haywire in the certificate chain.
It seems the file is only partially downloaded and permissions is wrong - I have seen this form time to time.
sudo rm -rf /var/tmp/pamac*
Disable the check for AUR updates in Pamac preferences - or edit the file /etc/pamac.conf and comment the setting.
$ cat /etc/pamac.conf
[...]
## When AUR support is enabled check for updates from AUR:
#CheckAURUpdates
[...]