Padlock on Subject sounds like something to do with encryption. If you hover over that lock symbol does it offer a tooltip pop up with info? Have you set up encryption in Thunderbird? Settings > Privacy & Security scroll to 'Email End-to-end Encryption'

Info on Encryption - in Forum Search type: Encryption and then select to filter by 'Thunderbird'. It should list a load of info on Encryption topics.

I do not use Encryption, so sorry not to provide more info, but hopefully it will get you looking in the right direction.

Hello! I took a look at the encryption settings but couldn't find anything to help with the problem. I unticked and ticked things, but the problem persists. It's so weird that it suddenly happened a year ago without me changing settings or installing extensions. The settings are still default.

Does nobody have an idea? It's so annoying to work with. To always have to copy and paste the addresses and open several windows to write an email, which could all be done with one click if it worked.

First check on the outgoing server (SMTP) entry in account settings that you have a default account selected.

Have you completely disabled all encryption settings for sending mail for the default identity?

Hello! Thank you for trying to help! I checked the default account and it was set already. Encryption was set to STARTTLS. Changing these settings don't get rid of the problem.

Edit: Very interesting. When I change the default program in the windows settings for the "MAILTO" to some other program, it always opens Thunderbird the same wrong way it does when it's set to the default program. The mailto doesn't even open another program. It's all very strange.