Hi @Julien_Palard.
Was the “from” account deleted? If not, the merge process didn’t complete.
I encountered a account merge hanging like this and reported my findings and the solution for the specific cause of the problem I had here:
Even if you are able to solve the problem, I do think the lack of communication about the error could be considered a defect (even if one that only affects a very small percentage of users). It is not a very friendly behavior for the admin to have to go hunting through the logs to discover that the process has failed and the merge is never going to finish.