Worked like a charm, thanks.
I remember having uninstalled / regenerated / reinstalled fiddler's root cert manually (in mmc.exe and firefox cert store) this morning, after the first time I noticed the error, with no effect though... not sure why, especially since firefox uses its own store, so fiddler's "Remove Interception Certificates" button should have no effect on it and the cert reinstallation process is the same I did before.
Well, I must have missed something!