Hi Stefan,
A few days ago I used a desktop with more power (more ram, more CPU). The 350MB file loaded slowly but all the pages appeared and I was able to split it. The 450MB file also loaded slowly (3,300 pages) and once it fully loaded the first ~2,000 pages went white on the grid (reloaded 2 times to test - same outcome), however if I click on the magnifying glass I could see them. I began to cut them in the desired file size, then the app crashed (wouldn't recognise any further cuts), loaded again still ~2,000 white pages however it let me cut the number of times required.
So essentially, I think its a performance issue.
Thanks
Michael