Seems strange, I finally resolved to use occ to list my external storages and delete the faulty storage.
In my configuration, I add 2 storages pointing to 2 differents drives hosted by the same server. Both configuration are exactly the same (except for the name of the share of course) but only one had the problem. I deleted it and nextcloud became available again. The remaining share was just not accessible, but it worked.
I’ll investigate the external storage config dump I made before deleting the faulty storage and report back…