I have this error too. This bug (I guess this is a bug) didn't happen in Knowage 6.x. (and unfortunately it's still happening in knowage 7.1.5 - I've tested today)
If you drag the dimension out of the OLAP table (to the top of the OLAP - What-If page, where we can set filters), than you will be able to show all the members of this hierarchy, and after that, when you drag back that dimension to the OLAP table (rows or columns) than the bug has gone and you can navigate through the hierarchy tree of members.
But it's annoying having to drag the dimensions out of OLAP table, expand its members and then return that dimension back to the OLAP table....
Does anyone else know what's happening and how to resolve this issue??
Cheers
Giovanni