There are many reasons you could receive a query failed message in ProClarity Professional. One recent customer found that when he combined members of a dimension in the background axis, he received, "Query failed:  The current query is not valid and cannot be displayed.  Create a new query by making different selections on the Setup Panel or revising your MDX statement.  If your query contains a calculated measure, the syntax may be incorrect and should be revised.  To restore a previous view, click the Back button".  "Warning  Query Failed!  (800F000A)". 

The customer could work around this by setting the solve order to > 0 or changing AGGREGATE to SUM.  This isn't optimal since users would have to do this manually and these settings would be lost when the next query is run. 

The issue is that the “dynamic” named set cannot be resolved because more than one member(s) is selected on a hierarchy that is referenced in the set definition using .CurrentMember.

The way my brain views this is that the combining (selection) of multiple members on the background blows up because, 1) SSAS finds all the named sets in the cube script which use that dimension, 2) those named sets are then resolved (dynamic!), 3) this named set uses .CurrentMember, and finally , 4) it makes no sense to combine multiple members and .CurrentMember.  I'm not sure I understand it either, so please feel free to set me straight or see this post -

Thanks to Steve Pontello for his invaluable assistance.