Hi,
Yes, this happens from time to time. In numerous instances, this has one of the following reasons:
- some images are too large (would not cause random behavior)
- the server is overloaded and throws random 50x or 40x responses
The latter might be the reason. Usually, thumbnails are created on demand and cached then. So the image calculation should not be a problem. But each image request is by default a PHP request. That alone can overload some servers. There is a solution: switch in the options to pre-rendered images and use the "thumbnail generator" to generate all the missing images. Unfortunately, you have to remove the .htaccess files in /images/eventgallery_generated manually to make the thumbnail images directly accessible.
If you try to open one of the failing images in a new tab, you might see an helpful error message.