What version of windows are you using? You mention other languages and XP, so I am wondering if any other computers are involved.

I just looked at the "Send to" feature in Windows 10, and despite my setting Thunderbird as the default mail app, it was still set to use "Mail" for email in the send to folder.

I used the procedure given here: https://superuser.com/questions/906463/adding-to-sendto-context-menu-in-windows-10 to add Thunderbird as an explicit send to item and that's working for me, inasmuch that if Thunderbird is already running, a new message composition window is created with the selected file(s) attached.

I don't think it would work for me if Thunderbird were not already running, as mine is set to ask for a Master Password before it runs.

But I don't know why if yours is still working, after a fashion, it would have changed its behaviour and stopped attaching the file.

This is probably related to the MAPI issue with TB 52, and you might find the installation of the MS VS runtime is helpful (XP, Vista,7,8).

In W10, to have the Send to/Mail recipient context menu command launch TB, TB should be set as the default in Control Panel according to the picture.

Thank you for your immediate responce. You gave me the solution. After installing the last "Microsoft Visual C++ 2015 Redistributable Update 3" everything was OK. I had a hunch that it was a matter of the OS in conjuction with TB. Thank you BK