I have 3 devices: 1. Desktop PC/Windows 10 with Thunderbird (TB) installed many years. 2. Laptop with TB installed many years. 3. Identical laptop with no TB and only a few apps that I use with external HDD for music tuition. Thursday early morning 11 Sept 2025 desktop working all ok, no laptops on. Thursday midday I receive the TB message "Enter your password for.... (my email address) on pop3....(my email ISP). I proceed as instructed but message "Login to server pop3......with username.........failed. Retry / Enter New Password / Cancel". I was using a VPN at the time, disconnected and tried again, same problem. Nearly 2 hours with the ISP techies since then, still no solution. Now it becomes interesting. With Desktop PC and laptop1 both shutdown, a clean TB installation onto laptop2 produced identical results. Techie reckons that an app somewhere is stopping the successful login from all devices. I have very different apps on all 3 devices. How can an app on my desktop PC (shutdown) for example effect TB on a new installation on laptop2? Techie also said that a full uninstall on desktop PC and laptop1 will not solve it. All updates are regularly done.It sounds like quantum entanglement has arrived here! Any thoughts.....?