Hey, thank you for still worrying about my problem.
I investigated the situation about complex matrices and found the trivial solution: hypre doesnt care about my complex matrix because the complex parts are simply cut off by using matlab2hypreIJ.m for creating the ij-formatted input files from the matlab matrix. Maybe there should be a warning about it.
Since i found out about this i am no longer using hypre, but SLEPc instead for the diagonalization of my complex matrix. As it turned out, the default method there (krylov-schur) is the fastest and less memory-conserving way for the diagonalization, without any need for a preconditioner.
Anyway, out of curiosity i would like to investigate about the ij-options you provided. What are they doing exactly? An interpolation? Sadly hypre isnt that well documented, so its hard to find out about options like these. In a free minute i will try the options and report about the outcome.