vat not calculated at regular price configurable product mag 2.1

[eaugustin]

See example below.

regular price backoffice 100 ex vat

Vat 20% discount 20%

Configurable product 

€ 96  

ex tax € 80 

Regular Price € 100 <- should be €120

Simple product  

 € 96  

ex tax € 80 

Regular Price € 120

how to fix?