我尝试更新操作系统，但收到以下消息：
$ sudo apt-get update
Hit:1 ttp://archive.ubuntu.com/ubuntu xenial InRelease
Hit:2 ttp://archive.canonical.com/ubuntu xenial InRelease
Hit:3 ttps://download.docker.com/linux/ubuntu xenial InRelease
Get:4 ttp://archive.ubuntu.com/ubuntu xenial-updates InRelease [102 kB]
Ign:5 ttp://dl.google.com/linux/chrome/deb stable InRelease
Hit:6 ttp://dl.google.com/linux/chrome/deb stable Release
Err:8 ttp://ppa.launchpad.net/hzwhuang/ss-qt5/ubuntu xenial InRelease
Could not connect to ppa.launchpad.net:80 (91.189.95.83), connection timed out
Err:9 ttp://ppa.launchpad.net/nathan-renniewaldock/flux/ubuntu xenial InRelease
Unable to connect to ppa.launchpad.net:http:
Err:10 ttp://ppa.launchpad.net/ubuntu-desktop/ubuntu-make/ubuntu xenial InRelease
Unable to connect to ppa.launchpad.net:http:
Get:11 ttp://archive.ubuntu.com/ubuntu xenial-backports InRelease [102 kB]
Get:12 ttp://archive.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Fetched 306 kB in 2min 8s (2,387 B/s)
Reading package lists... Done
W: Failed to fetch ttp://ppa.launchpad.net/hzwhuang/ss-qt5/ubuntu/dists/xenial/InRelease Could not connect to ppa.launchpad.net:80 (91.189.95.83), connection timed out
W: Failed to fetch ttp://ppa.launchpad.net/nathan-renniewaldock/flux/ubuntu/dists/xenial/InRelease Unable to connect to ppa.launchpad.net:http:
W: Failed to fetch http://ppa.launchpad.net/ubuntu-desktop/ubuntu-make/ubuntu/dists/xenial/InRelease Unable to connect to ppa.launchpad.net: ttp:
W: Some index files failed to download. They have been ignored, or old ones used instead.
令我感到困惑的是，当我尝试从终端使用“ping”方法时，服务器似乎工作正常：
$ ping ppa.launchpad.net
PING ppa.launchpad.net (91.189.95.83) 56(84) bytes of data.
64 bytes from haetae.canonical.com (91.189.95.83): icmp_seq=1 ttl=38 time=474 ms
64 bytes from haetae.canonical.com (91.189.95.83): icmp_seq=2 ttl=38 time=497 ms
64 bytes from haetae.canonical.com (91.189.95.83): icmp_seq=3 ttl=38 time=520 ms
64 bytes from haetae.canonical.com (91.189.95.83): icmp_seq=4 ttl=38 time=542 ms
64 bytes from haetae.canonical.com (91.189.95.83): icmp_seq=5 ttl=38
谁能帮我弄清楚这里发生了什么事？