session_start(): Failed to initialize storage module

You can't unless you're the admin. If that's the case, on Linux you can use sudo.

But I'm not sure I understand your problem with Putty. I never heard of problems with ppk file. Putty is just a .exe, there's no need for administrator's rights.

And I missed the part where you are using AWS. Check which user is the owner of the files and directories, and check which user run the web server. Depending on your setup you may have the web server running under the "apache" user, or something else, and the files be owned by you. If that's the case, you need to give the write permission to the group, not just the owner with chmod 775, instead of 755. And make user both users are member of that group (user owning the files and user running the web server process).