We decided to change the reputation system. 

All previous reactions give now zero reputation points (they are now neutral) and a newly created reaction is the only one that gives reputation (positive).

For that reason, a recount with the new value for each reaction is necessary, but it does not work.

Do do it, i go a membre profil in the AUI, select a random member, click recount, and select "recount for all members"

I use the latest version of IPS4 (4.2.7)

Any idea how I can solve that problem?

Did you confirm the recount has actually completed (a background task should be launched and you can monitor its progress on the AdminCP dashboard)?