I have found an solution for the problem, you can find it in a few other posts in this forum. Everybody gives the problem another name, so it is hard to find out you are having the same problem.
This was the trick: put an entry for “height” in the stylesheet. First you have to find out which stylesheet you are using. I am using the ngg_dkret3.css. Then find .ngg-gallery-thumbnail-box. I am using 120 pixels height. But how much you actually need, you have to try, because this will be different for most sites.
/* ----------- Gallery style -------------*/
.ngg-gallery-thumbnail-box {
float: left;
height: 120px;
}
But what if nextgen is updated the next time? If there is no solution in the next update, backup your stylesheet before update or upgrade.