unable to update or install plugins

OK, this is weird: I cannot update or install plugins and I have tried just about everything out there that I can imagine. For updating, I get a success message but then when I refresh the page it goes back to not being updated.

For install, I just get that duplicated folder message (or can’t create the directory, or not writable…) None of that is true. I have the correct permissions for all folders, and for jetpack I have never even installed it so it is not possible for there to be a duplicate (although I have looked anyway.)

I have tried all sorts of variations with setting the tmp file in config.php: setting it to be wp-content, or uploads, or a tpm directory with 777 permissions) in wp-content. Nothing. I finally called my isp and they are clueless concerning the problem.

Does anyone have ANY ideas? I am sure it must be something simple…should I re-install the WP core? And, if so, anything to be on the lookout for there?

Many thanks in advance,
Debbie