If you drag it around you should be able to dock it, you'll see the panel behind it turn blue if it finds a target.

FME like a lot of different software uses dockable and stackable window panes.

If you hold and drag on the pane Title Bar, and drag it onto the edge of the FME window, you should get an outline of where it will dock onto one of the window edges.

This video is from Revit, so not quite FME, but same basic concepts shown in 2 minutes:

https://www.youtube.com/watch?v=DleiMs0n9Mk

Tried this but without succes.

Then fall back solution is to reset the layout back to default. The solution involves resetting/deleting the relevant Windows registry keys as described in this post with similar circumstances:

https://knowledge.safe.com/questions/85059/restore-translation-log-window.html

This did the job! Thanks for the tip.