Since upgrading to J! 3.9.0 I'm getting this error on EVERY extension update except for the few who use an update routine that appears to bypass normal Joomla workings - StackIdea's EasyBlog and JoomlaShine's Uniform are 2 that will still update OK.
However, everything else (ALL of which have happily updated from the Update area in the past) fails. The extensions are all 3 types (component, module, and plugin) and are both paid and free. They all require manual installation to update.
I checked my /tmp path and it's correct and writable. configuration.php is, as I would expect, unwritable, but I saw in a different post some else's Folder Permissions screenshot showing it as writable so I changed it to 777. Still fail to install anything.
As I manage some 80 odd sites, this is a major time waster. Is anyone else experiencing this?