I receive this error message when loading more than 41 100*100 terrain files. But I'm quite sure that the engine and my system should be able to handle much more terrain, as the framerate is still very high (~300 in windowed mode) when having loaded already 40 terrain files. So what's the problem with having that much terrain files?

What does your video memory consumption look like?
Do you use shared textures?

I have a great increase of bitmap memory consumption and that's propably because I recreate the first two skins of each terrain file. But this should be capable by the engine as well, as every terrain file I load could have this amount of skins as well which would have to be loaded in this case too.

EDIT: some more info:
I have now decreased the bmp video consumption by 150%. This changes nothing. Referring to the statistics panel I should have more than 600Mb free memory for video consumption, so I don't see a lack of memory here. It seems that I receive the error in the moment I have more than 100k triangles visible per second.

You should be careful with the number A7 gives you. I had a system where some reserved ram area for dumping currently unused bitmaps was counted as available video memory.
Check the specs of your graphics adapter to be sure.

Might be another d3d related limitation, though.

A7 reads the free video memory amount from the DirectX driver. It's theoretically possible that the driver gives a wrong number, but the numbers I've seen so far were correct.

Video memory is consumed mainly by textures meshes.

The E_OUTOFMEMORY error message is not from the engine but from DirectX, so maybe the reason is something else. I've seen wrong DirectX error message before. It would be useful to know which command causes the error message.

I have received this error as well. Although, I thought that by the time I switch over too .dds there should be no problem. However, I don't understand why it would appear and crash the program, as I have sufficient memory.