Solution : https://service.sap.com/sap/support/notes/126702 (SAP Service marketplace login required)
Key words : 
customizing financial accounting / financial accounting global settings / document / document header / define validations, exchange rate eur requires ongoing maintenance, newly defined exchange rate type xecu, newly defined exchange rate type euro, user-defined exchange rate type, -defined exchange rate type, financial accounting, newly posted open item, exchange rate type xecu, exchange rate type euro
Related Notes : 
       
| 169631 | Currency Customizing for euro to Release 4.6 | 
| 126411 | Special topics in Releases 4.0A and higher | 
| 124593 | Special topics in Releases 3.0/3.1 | 
| 124405 | Exchange rate type EURO in Releases as of 4.0A | 
| 124018 | Exchange rate type EURO in Releases 3.0/3.1 | 
| 123819 | Exchange rate type EURX in Releases as of 4.0A | 
| 123696 | Exchange rate type EURX in Releases 3.0/3.1 | 
| 123467 | Exchange rates and recommendations for ratios | 
| 120420 | Collective Note on Customizing of currency tables | 
| 116073 | Legal regulations and translation logic (old/new) | 
| 115016 | EMU: Customizing of translation tables as of 4.0A | 
| 113783 | EMU: Customizing of translation tables in 3.0/3.1 | 
| 106969 | Can Customizing settings be inverted | 
| 91481 | Customizing of currency tables for the euro |