Hi,
I'm the mirror.unlockforus.com maintainer and I can tell you that the mirror is up-to-date. There are two servers in EU serving packages in that mirror via Anycast, load balanced. I'm pretty sure the speed of these two servers serving our EU Mint community users on that side of the globe is pretty fast (both servers can serve simultaneously as requests come in - specially if someone is using something like apt-fast), in addition to its synchronization. mirror.unlockforus.com should be one of the fastest mirrors to get updated as updates are pushed to the master mirror (packages.linuxmint.com).
Also, it seems you have the same problem regardless the mirror used. Maybe it was a bug in your install (something that nobody else is experiencing). Please check your installation and try it again. If you or another user in the community happens to have problems with
http://mirror.unlockforus.com (EU), please do let me know whenever possible (you can email me at the address there mentioned). We want to make sure we always have good mirrors for our Mint lovers

All the best to this community and greetings from the other side of the globe!