For now I updated the Whitelist in Cerber Security as stated @ Known Plugin Conflicts. Everything is now green but the 403 is still displayed. There is too little chance that it’s a server side issue sine my other sites are on the same server without that issue. This site is the only one with Cerber installed.
My actual question is then to know if there is some delay to update the status code in MainWP, since any new data sync doesn’t change anything, even with Cerber deactivated.
Regards.