Hi Sergio,
we encountered this problem after changing the password of the BW-user that was used to sync the BW-roles with BO. We also changed the password in the SAP authentication menu in the CMC and the login to BO with a BW user worked perfectly fine.
But when we tried to create a new query in Dashboards based on a BEx-query the error occured after selecting the connection (when the list of BEx-queries should be loaded).
After simply restarting the two Dashboard servers the error message was gone. I think there was a problem with cached data...
Maybe that also works for you?
Jens-Uwe