Hi Nadine,
As you know, BuddyBoss contains a basic version of BP Profile Search, so you have a choice between using the built-in version or the public BP Profile Search plugin; using both at the same time causes problems as in your case.
I suggest one of the following solutions:
1) To use the basic built-in version, deactivate the BP Profile Search plugin;
2) To use the public BP Profile Search plugin, deactivate the built-in copy; go to BuddyBoss -> Settings -> Profiles and uncheck the checkbox Enable advanced profile search on the members directory.