Sorry to open a new ticket but you closed the other one and I could not answer there.
If by upload and install, you mean intall the package manually using extension installer, that's what I usually do but I can't on that host as I get a 403 for the file administrator/index.php.
In fact I tried the automatic update only because I could not manually install and that means actually none of the two method are working.
Do you have an idea what could cause this 403 ? The certificate issue would not matter for me if I could install manually.
As said in previous ticket, even setting concerned file and up folders to 777 does not solve. Host support say that Admintools is causing this 403 but I have a clone of this website with same Admintools settings on another host and no problem at all, so I know it's not the reason.
Thanks in advance for your help.