I've Googled this question in a large amount of sites, but I've not found any good solution.
The problems seems a common one, on the web. These are the details...
If I try to upload a file directly in the Media Library section, Chrome console shows two calls to /wp-admin/async-upload.php file: first one returns a success 200 reply (and the file is uploaded), second one shows a 303 forbidden reply, so the file is not shown.
Reloading the page, my picture is there... what a mistery!
The real problem comes when I try to attach a featured image to a post: the libray does not show up! Well, the window pops up, but the loader keeps on spinning, and no images is shown. If I try to upload a new image, I receive a WP error, telling me that the upload failed to complete. This time Chrome console shows several call to admin-ajax.php, no JS/PHP errors but no results.
I tried several tricks, like modifying the first lines of async-upload.php to make AJAX always active, but this is not the solution.
Image files, pdf files,... no difference! The problem is still there.
Server log file does not show anything relevant to me.
Any good hint?
Thanks
Wordpress v4.9.7 (same problem with prev version)
EDIT
I've noticed that a similar issue rise up when I try to update a plugin. The update seems to fail, an error message tells me the following
Update failed: {"success":false,"data":{"update":"plugin","slug":"login-recaptcha","oldVersion":"Versione 1.2.3","newVersion":"","plugin":"login-recaptcha\/login-nocaptcha.php","pluginName":"Login No Captcha reCAPTCHA","errorMessage":"L'aggiornamento del plugin non \u00e8 riuscito."}}
But, refreshing the page, the plugin is correctly up to date!