WebGL texture red and flickering on Apple devices

I'm currently in the progress of writing a simple WebGL application, where multiple textures are blended on top of each other.

The application works fine in Chrome, Firefox and IE on Windows, and also works on Android phones.

On Apple devices, however, the textures are red and flickering in any browser. I tested this on Safari on iPhone or Chrome and Firefox on a MacBook.

For example, the desired result is a wooden texture:

However, on Apple devices, this is shown:

There are no errors and no warnings from WebGL. What could be the issue or a hint to the issue here?