Great information from d4m1en on https://redd.it/k104iy:
Step 1: write to customer service
SOGECAP
Service Relations Clients
42, boulevard Alexandre Martin
45057 Orléans Cedex 1
Phone 09 69 362 362
Step 2: write to customer complaints
Sogécap Réclamations Clients
42, boulevard Alexandre Martin
45057 Orléans Cedex 1
Step 3: write to ombudsman
La Médiation de l’Assurance TSA 50110 75 441 Paris Cedex 09
http://www.mediation-assurance.org/Saisir+le+mediateur
Ombudsman will ask for evidence of the two previous steps and at least 2 months without satisfactory answer, before taking your case.
The same 3-step process is described on https://www.assurances.societegenerale.com/fr/reclamation/ (mirror). Note that all French financial institutions are required to display the recourse processes somewhere on their website.
In parallel, one can message the Fisc via one's account on https://www.impots.gouv.fr/portail/ to inform them of the issue.