The output of the commands './db2_monitor' and 'ldd db2_monitor' ran in the db2 probe directory points towards a missing library:
'./db2_monitor: error while loading shared libraries: libdb2.so.1: cannot open shared object file: No such file or directory'
This library libdb2.so.1 is located away from the other libraries. Therefore, the environmental variable within the Robot controller needs to point to this location for the probe to activate.