Hi @abalbuena,
There’s some invalid UTF-8 content in your database that’s causing this. Double-check that your database character set configuration is correct (both in the the database, and in your config.inc.php); to locate the invalid data, the quickest way is probably to temporarily edit lib/pkp/lib/vendor/slim/slim/Slim/Http/Response.php near line 353 and dump the $data that couldn’t be successfully passed through json_encode.
Regards,
Alec Smecher
Public Knowledge Project Team