@rrvdbeek – What has changed on your site since this issue started? There could be a plugin conflict going on here or at least it should be ruled out. Have you tried deactivating all of your plugins, clearing your browser cache (and site caches if you are using a caching plugin), then reactivating the NextGEN plugin?
If this fixes the problem then it was likely one of the deactivated plugins that was causing an issue. Now, retrace your steps activating each plugin and re-checking to see if the problem returns. If it does then that last activated plugin was probably the one. Keep reactivating and in most cases it is also best to keep checking after each plugin, sometimes there may be more than one plugin on a site that will create an issue. If you do find one (or more) plugins causing this problem we would greatly appreciate your feedback on them so the developers may address them as soon as they can.
Also to note, in rare cases it may be the theme itself that is causing a conflict so while you have just the NextGEN plugin activated you might consider temporarily switching to one of the default WordPress Twenty series themes and see if this corrects the issue as well.
If the problem still persists after all of this we would still want to know, even more so, as we will still want to sort out the issue and may need additional details.
Thanks!
