Môrena
Something bad has happened with regard to xvi image processing (in 6.0.2 and maybe a little earlier but I have not tested).
A friend attempting to get it running in a Windows 10 VM (from a Mac host I think) asked me about this. Unfortunately I have the same experience with a standard Windows 10 installation and Therion 6.0.2, over a couple of projects, so I’m assuming it is universal.
Opening th2 files via XTherion Map Editor that include background images (jpg, png) seem to be OK.
Opening some th2 files with that include background images display partially (survey legs and splays only, no stations) but are not listed in the background sketches pane on the right.. Opening other th2 files the xvi is invisible and also not listed in the background sketches pane on the right.
Attempting to insert a new xvi into the former th2 gives an error as below and as attached (from different projects and machines).

Opening the latter gives the image below – note that the background images pane is blank in both cases.

The content of the header in the th2 file seems to be OK, as highlighted below. It is just that XTherion is now silently failing to display it (or failing to display it properly).

Seems like this is a bug!
Bruce