I left synchi with four files open. When I restart synchi, the four files get reopened and the first tab and file gets selected.
Now, when I click on i.e. tab 4, the tab will be activated but it shows file 2!
It keeps showing file 2, even when I click tab 3 after. Only when I click tab 1 or 2, and so on ...
Finally I found out ...
When you open synchi with multiple tabs from the last session, you have to click all tabs in the shown order, to "activate" them somehow. Not until this action, you can be sure to see the correct file that belongs to a tab.