Hello

Since the upgrade to 1.4.5.4, randomly thumbs are not loading and also random full images will not load. This also now with 1.4.5.5. Both Firefox as well as Internet Explorer are giving this. Forced loading with Ctrl/F5 does not help. Site is http://www.3dart4u.com , theme is a default Zenpage theme. But I doubt that it has to do with the theme: even when I am logged in as admin in the ZP-core, thumbs there are also randomly missing in there.
Tried a database refresh, no effect. Tried metadata refresh, no effect.

There must be an error somewhere. Also review:
http://www.zenphoto.org/news/problems-with-albums-and-image

And check permissions
http://www.zenphoto.org/news/permissions-for-zenphoto-files-and-folders

@ acrylian
Permissions are set correct.
And I do not have large images on my site, not in filesize or size (max 1920 x 1080 pixels).
Tried the Obscure Cache Filename, Protect Image Cache and so on, but no effect.
Also did the "debug on missing image" but it shows the thumb and it's properteis just fine then.
My site worked fine but ever since the 1.4.5.4 upgrade, this missing thumbs and images has started.

Update: I enabled Imagick and now all seems to work fine. Never had to use that before?
Downside is that it's all very, very slow loading now?

Imagick should be faster than GD but that all depends on your server. It could be very well that your server has changed. I did notice any issues with several upgrades recently.

Did you clear the cache? Slow loading happens of course if the cached images are created on the first view. Please see the user guide about how caching works.

Depending on your server config 1920 x 1080 images can very well be too much