Slitherine

On the attached screenshot German fighter cant rebase to hex NE of Bruges, but its technically clear hex and air units should be able to be based there.
My bet it that it has something to do with adjacent resource hex being controlled by enemy. On the other hand hexes near Liege are fine.

My guess is this screen shot was taken the turn right after capturing Amsterdam during the German player's turn. The hex is considered to be Holland's Hex not Belgium or German.

After the German's turn the hex will have converted and able to re-base there. I do not consider this a bug.

Buth thats grey hex, it looks German controlled.
Ones near Brusseles, for example, were captured this turn too, but air units can rebase there.

I wonder if this has something to do with the hex being adjacent to Bruges which is Allied controlled. I think hexes adjacent to resources are checked before clear hexes deciding whether you can base there or not.

We need more examples before we can make a final conclusion.