I'm running SIG Pro 2.5.5 on a vanilla J! 1.7 install.

I cannot get the Greybox lightbox to show my photos properly. When I click on a gallery picture, the lightbox opens, but it shows only a spinning "loading" logo. The other lightboxes works perfectly - but I only want Greybox!

And sometimes, the gallery shows empty white boxes instead of thumbnails.

I had the same gallery folders in a J! 1.5 + SIG PRO 1.2 install, and it worked flawlessly.

This is probably a script conflict with your template...

I wouldn't think so, because it works "sometimes", but most often not.

Also, some galleries' thumbnails are blank - and others work fine.

The other lightboxes work, only Greybox gives me these errors

What I mean is that this pop up engine may conflict with your template or another extension you use. If you cannot find what cause the conflict and disable it just use another pop up engine.