Module get's stuck when loading images

My client is a faculty of a university. They use Captivate to create e-modules for part of the training.
While going through a module, this sometimes seems to go completely well, but sometimes it doesn't. Certain images are not loaded correctly (see screenshot).

My client is a faculty of a university. They use Captivate to create e-modules for part of the training.
While going through a module, this sometimes seems to go completely well, but sometimes it doesn't. Certain images are not loaded correctly (see screenshot).

Does anyone haven an idea of what is happening here and have solution?

PS. I tested the modules locally and on an other web server, same results.