I am no expert but I think Gallica is open to anyone , there are no secrets in Gallica that should be kept " for french eyes only".
But your computer may protect you strongly from foreign sites, may be you could either:
disconnect your VPN
or
erase the bnf cookies on your computer
or
try another navigator
and surely some other answers , but as I said ,I am no expert.
I found these solutions on the web by entering "accessibilité du site gallica depuis l'étranger""problème d'affichage sur Gallica" " faut-il un VPN pour accéder à gallica depuis l'étranger".
You are not the first one to have this problem and I would really lke to find an answer.
Of course I could send you the maps of Le Havre if nothing works .
Good luck , let us know if something works.