what exactly does the error say? something about nstemp by any chance?

I did more testing.... The error only seems to happen when I copy an image from my paint program (PSP 8), as I just copied a image from a website and it did allow me to paste it into an email reply.

So very strange, as coping images from my paint program and pasting into email is the way I've always done this for years..... sending screen shots to help others see what I'm seeing.

If I save the image from my paint program, and then insert it I get a broken image rectangle, and the same error message.

So I tried the 'paint' program that comes with Windows - same error with both direct pasting and with proper image inserting.

The error message says - Sending of the message failed. There was an error attaching kbdockifdinnfmdg.png. Please check that you have access to that file.

One last test - I tried any image from my saved graphics folder. I have sent these images many times via email over the last 25 years. And now since this update I cannot share them via Thunderbird any more.

This email feature is important to me.

Any other test you want me to try?

Like my comments about nstmp, your reply reinforces my feeling that the error is coming from the file being lock by another program. Most probably your anti virus product.

I am guessing your anti virus has the file locked while it scans the file in the temp folder. Try disabling whatever "on access" scanning you can and see if it helps.

Alternatively you can restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.