I'm afraid there I have a problem. For the site in question (hanvec21.fr) I do not have access to the server. (It's for that reason we are looking to migrate the site onto a different host). So I'm not sure how to access the config file without asking the guy running the server to intervene. I can do that, but he is slow, very slow.
Is there anything else I can try?
By the way - I installed your plugin on another site I'm in the process of migrating and it worked fine, using Firefox. I was able to upload the 3.9mb plugin from UpdraftPlus. Makes me wonder whether the problem is not server related.