Thanks again!
OK, I just downloaded the current version of WordPress, unzipped the file, uploaded the wp-admin and wp-includes folders to my site via ftp.
Exited WordPress, re-entered WordPress.
No change. I get the same error message when I search for any plugin from the dashboard.
Thanks for quick reply.
Yes, I have done both (deactivate all plugins, switch to default theme). The problem remains in both cases.
