Misleading cell security error message prevents drillthrough action on SSAS2008

I have a cube with no cell security configured, and get an intermittent problem with a drillthrough action. The cube has two measure groups: One with accounting transactions from a financials application (Oracle Financials) and one with a set of FX rates. The FX conversions are done with a measure expression, and forced to leaf level by the cube script. This problem is taking place on SSAS2008 (R1).

scope ([functional currency].[functional currency].[all]);
    scope ([measures].[herp fx rate]);
        this = null;
    end scope;
    scope ([measures].[derp fx rate]);
        this = null;
    end scope; 
end scope;

scope (leaves ([functional currency]));    
    scope ([fx period].[period].members);  
        scope ({[measures].[Herp GBP]});    
            this = [measures].[accounted] * 
                   [measures].[Herp fx rate];    
            //fore_color (this) = 4227327;   
        end scope;    

        scope ({[measures].[Derp GBP]});    
            this = [measures].[accounted] * 
                   [measures].[Derp fx rate];    
             //fore_color (开发者_运维百科this) = 32768;  
        end scope;  
    end scope;     
end scope;  

Calculate;

The cube has a drillthrough action on it. When I attempt a drill through action (via the SSMS cube browser, for example) the cube intermittently returns an error to the effect of

The drillthrough operation cannot be performed because cell security does not allow access to the source data

Various forum postings suggest that setting IgnoreUnrelatedDimension=False will fix this. However, I can't find any documentation that explains why this happens and the error message is somewhat unhelpful. Does anybody have a coherent explanation of what is causing this error?