I have encountered a problem that is similar to yours, with the Ubuntu Server installed in a VM, but the underlying cause should be different. I put out the problem description and the solution in case that someone who encountered the same problem reaches here.
Brief Summary:
The similar problem is caused by the network condition of our office. When the problem occurs, I used a bridged network for Internet access. After changing the VM network setting to the normal NAT, the problem is mitigated.
Background:
I have installed Ubuntu Server LTS 18.04.3 with VMWare Player. After the installation is completed, I have used the VM for several days, including upgrading the system with sudo apt update|upgrade and install new applications with sudo apt install <appname>.
Problem:
After a weekend, I reopen the VM and want to install some new software. So I first try to update the repository information with sudo apt update to see if there are something that is upgradable. However, after executing this command, I get the following results:
gary@ubuntu-vm:~$ sudo apt update
Ign:1 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic InRelease
Ign:2 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates InRelease
Ign:3 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports InRelease
Ign:4 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security InRelease
Err:5 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic Release
Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown. Could not handshake: Error in the certificate verification. [IP: 101.6.8.193 443]
Err:6 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates Release
Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown. Could not handshake: Error in the certificate verification. [IP: 101.6.8.193 443]
Err:7 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports Release
Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown. Could not handshake: Error in the certificate verification. [IP: 101.6.8.193 443]
Err:8 https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security Release
Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown. Could not handshake: Error in the certificate verification. [IP: 101.6.8.193 443]
Reading package lists... Done
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-updates Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-backports Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'https://mirrors.tuna.tsinghua.edu.cn/ubuntu bionic-security Release' no longer has a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
which is similar to the aseked problem(e.g., Ign:3 and Err:5), but not the same.
Solution:
I have searched the related topics on Google, and many said that the problem is caused by incorrect configuration of certificates. However, I should never change any certificate configuration after installation of the system. Besides, avoiding certificates authentication should not be a regular routine.
To make sure that I did not change related configurations, I reinstall the system. I found that the installation cannot be completed, with the error log similar to the above one. After finding this, I guess that this problem should be caused by the network connection problem, as in this point there is no configuration made to the system.
Therefore, I checked the configuration of the VM instance, and found that this VM uses a bridged network rather than NAT. So I changed the network setting to NAT, which is usually the default network setting, then everything returns to normal!
After that, I recalled that when I first install the VM, I connect my computer to another computer to share the network (using NAT at the second computer). Later, I have my own network connection and I want the VM direct access to the physical network, so I changed the VM network setting to a bridged network, which then caused the problem (It's simply a network connection problem, because the physical network require authentication for network connection, while the VM does not have the credentials).
