What’s wrong with my XHTML?

I think you have removed some tags.
The default index has this:

< ! - -
<?php trackback_rdf(); ?>
- - >

And my guess is you removed the top and bottom lines ?
(I’ve had to space them here to show up)

I removed the comment declarations in fact. Now, I’ve added them again, but the page still doesn’t validate as XHTML 1.0 Transitional.
What else could I change to make the page valid?

They aren’t there, or they are not there properly.
It must be < ! – – and – – > with no spaces between those characters.

Staticize Reloaded caused the problem: I had changed the source code but the validator had loaded a cached page. Now, the page is valid again:
https://validator.w3.org/check?uri=http%3A%2F%2Fwww.southernapproach.ch%2F
(and I’m thinking of deactivating Staticize Reloaded)

It is recommended that you do deactivate staticize reloaded while you’re tweaking your site, and flush the cache when you’re done.