It sounds like you have an Anti-Virus product scanning either incoming mail or any downloaded attachment/document or both.

Thanks for your reply. Yes, I have ESET but I've had that running with Thunderbird for ten years with no problems. I dare not risk switching it off.

You can safely turn Eset off - in fact you don’t need any antivirus software at all when running Thunderbird, unless you don’t have a router / home LAN and your machine is connected to the internet directly. But even then it would affect other applications or the machine itself more than it would do for Thunderbird. Not to start a discussion, you can read more about the pros and cons here.

Disabling and sometimes rebooting or even uninstalling such software is also required in order to rule out "external" culprits, and even the best security software can have flaws and cause issues with Thunderbird or any other email client or other application.

Regardless of the above, the least you could and perhaps should do is disable Allow antivirus clients to quarantine individual incoming messages in Thunderbird’s Options > Security panel > Antivirus tab.

Additionally, make sure to disable email related features in general security software - typically special AntiVirus software does a better job than Internet security suits where that may be a part of.

If none of this helps, make sure the Thunderbird version is exactly similar to the one on your other machine.

Can you check for improvements and report back if anything helped?