Alright, so here's what I've done:
- built release 3.6.9, same problem though
- built debug 3.6.9, which gives me the following output in the log-file:
Under the OpenGL-section:
Using extension "GL_ARB_texture_compression".
Unable to find extension "GL_EXT_texture_compression_s3tc".
Can use compressed textures: NO
Texture compression available: YES
And from there on, it gives me a bunch of messages whenever trying to load a texture:
DDS ERROR: Couldn't open '60mm_main.dds' -- DDS is compressed but compression support is not enabled
until it finally quits with the same message.
I modified the source-code slightly to see what would happen, if the texture compression was enabled by setting the "Can use compressed textures"-flag to always be true. After compiling this build, I could start a campaign without any problems, but the textures were just plain white.
So, I think my Linux graphics driver just doesn't support all required features (unless there's some way to disable the use of DDS compression in the config-file!?).
Still, the good news is, I just downloaded the executable and ran it under windows, where it works just fine. Oh well, would have been nice to play it under Linux, but whatever...
In any way, thanks for your help!