I have checked Image Addon Lightbox on. It seems okay to me. Can you please share a page link to investigate?
Hi, Ofi. Just check it out in different browsers. For example in Firefox. And reload the page several times, the result may be different each. Investigation is not required, I know the reason and will inform you for elimination.
This is because the "+" character of the default font is used. Browsers render the same font and line-height differently and this causes alignment issues.
The solution will be to replace the "+" with some icon from Font Awesome or the ability to upload user own icon (image) for this.