Broken installation of GnuCash

[Daniel]

Ok, something is wrong with my computer, but I don't know what to do. 
The following error occurs on *three* different versions of GnuCash:

----//----
Go to Reports > Assets & Liabilities > Asset Barchart

=> The last bar is clearly wrong. The bar is like 1/6th the size of what 
it should be. It also displays the error:

;;; WARNING (gnc:get-commodity-totalavg-prices:  Sorry, currency 
exchange not yet implemented: EUR 574.65  (buying  GBP 400.00 ) =?  $0.00)
----//----

Tested on versions: 1.8.12, 2.0.5 and 2.1.4 (downloaded today).

HOWEVER... if I take my GnuCash file to my girlfriend's computer, on 
*her* GnuCash (2.0.5) the bar is just right.

I don't understand what's happening. It looks as if the problem is 
outside GnuCash itself, like some broken library or a broken 
configuration (I tried moving ~/.gnucash to ~/_gnucash but that didn't 
help).

Any ideas?

Best,
Daniel.