I am having a problem with my email program, Thunderbird. Up until 2 days ago, this has been working flawlessly for years, until it suddenly stopped sending or receiving emails. My initial thought was that a setting had been corrupted somewhere, but I checked everything (IMAP, SMTP settings etc.). I contacted my ISP, and confirmed with them that my settings were correct, username and password (I can still log into my web mail just fine) etc. Everything is as it should be. I have tried uninstalling Thunderbird using apt remove, and opted to install the Snap version instead, but still, Thunderbird absolutely refuses to communicate in any way. I have noticed that even if I go into Thunderbird Help, and click on one of the links there, they are instantly blocked too!
As a sanity check, I also have another laptop running Linux Mint. I installed my profile on Thunderbird on that machine, and in a matter of minutes I had my email running (exact same user credentials, ISP, home network, email account). I am therefore thinking that this has got to be an OS problem on my desktop. This is no longer looking like a Thunderbird/Profile/ISP/Network issue.
How and why is Thunderbird (version 91.5.1 64bit) suddenly blocked from the Internet on my Ubuntu 20.04.3 LTS (x86-64) based desktop? Is there a firewall issue I am not aware of perhaps?
Update I just tried again, and this is still not working! See the screenshot, where I have attempted to use the auto-config option when setting up a new account in Thunderbird. The exact same process works without a problem on my laptop on the same network, and the laptop is running Linux Mint...