I have forgotten to check dlerror. It says:
/usr/local/lib/libbassflac.so: undefined symbol: BASS_GetVersion
I am using Ubuntu Linux 64bit with the lastest version of bass. Tried now to load BASS_GetVersion via LOADBASSFUNCTION, but it doesn't helped.
After loading BASS_GetVersion via LOADBASSFUNCTION a call returns 33818880. But shouldn't it return something like 0204090X for version 2.4.9.X?
Found the solution. libbass.so must be loaded with RTLD_NOW | RTLD_GLOBAL instead of RTLD_LAZY. Now the plugins also works fine.