To anyone facing the same or similar issue:
Check oracle user's environment variables. I was able to install successfully, but had to write in .bash_profile the following:
PATH=$PATH:$HOME/bin
PATH=$PATH:$HOME/bin:/oracle/ISP/112/bin
ORACLE_HOME=/oracle/ISP/112
ORACLE_SID=ISP
ORACLE_BASE=/oracle
GRID_HOME=/oracle/GRID/11203
LD_LIBRARY_PATH=/oracle/ISP/112/lib
export PATH
export ORACLE_HOME
export ORACLE_SID
export ORACLE_BASE
export LD_LIBRARY_PATH
Also, the environment variable ORACLE_BASE is supposed to be /oracle/SID (recommended by the oracle white paper referenced by SAP in SAP on Oracle Real Application Clusters (RAC))
But it only work for us with ORACLE_BASE set to /oracle
Good luck,
Fabian