I have been using Calibre to convert Amazon ebooks successfully for some years.

Today I have downloaded the latest versions of Calibre and the plugin conversion tools, and restarted my computer.

Now Calibre can not convert my books. When adding an ebook from Kindle PC, it comes into Calibre as a number, not a named book (as per normally happens).  It says it is in AZW format but it won't open.  When I try to manually convert it, it tells me it is unable to convert a KFX format.

I have checked that the plugin has successfully been added.

Can anyone help, please?!