No instant preview anymore for tikz pictures in LyX; reset previews?

Question

I'm inserting figures into my LyX document by adding a graphics float, then an instant preview window inside the float, and a tex code window inside of that preview window. Here I put my tikz code. It has been working fine until I changed the code of one figure. Since then, all figures are not anymore shown as previews in LyX. I undid the changes, deleted the LyX temp folders, but previews are not showing up. Compiling the document, however, works just fine.

When I slightly modify the tikz code of a figure, e.g. by adding a % at the and of any line, this figure can be previewed again. I'd rather not add a % to every figure in the document - is there any way to reset all previews in the LyX document without deleting the tikz code?