as said previously, the right place to ask for help is my support platform
6 months has passed from the thread opening and 4 months from the last answer. If you take a fast look to the MediaGrid changelog you'll see that I've made a lot of improvements and fixes.
This error is the most common but I cannot use a patch because depends on your servers. Timthumb is probably the best script to manage images, but requires some permissions on the server folders.
Anyway is not the only cause. In 6 months I've seen a ton of different situations, so please open a private ticket sending me a direct link to a thumbnail