On my system, I have LADSPA plugins distributed over several places, e.g.
Accordingly, my environment variable LADSPA_PATH is set to "/usr/lib/ladspa:/usr/local/lib/ladspa" (without the quotes).
LinuxSampler doesn't like multiple paths, though - it states at startup:
Registered internal effect systems: LADSPA
Could not scan LADSPA effects: library path '/usr/lib/ladspa:/usr/local/lib/ladspa' doesn't exist
Registered internal effects: 0
As it seems, this should be either handled in src/effects/LadspaEffect.cpp or src/common/DynamicLibraries.cpp.