Check to make sure that the po_plugins database table exists. Check your server logs for database errors. Also check that the database table has the right columns.
This is the table structure:
pl_id bigint(20) unsigned NOT NULL AUTO_INCREMENT,
post_id bigint(20) unsigned NOT NULL,
permalink longtext NOT NULL,
permalink_hash varchar(32) NOT NULL default ”,
permalink_hash_args varchar(32) NOT NULL default ”,
post_type varchar(255) NOT NULL default ”,
status varchar(20) NOT NULL default ‘publish’,
secure int(1) NOT NULL default 0,
children int(1) NOT NULL default 0,
pt_override int(1) NOT NULL default 0,
disabled_plugins longtext NOT NULL,
enabled_plugins longtext NOT NULL,
disabled_mobile_plugins longtext NOT NULL,
enabled_mobile_plugins longtext NOT NULL,
disabled_groups longtext NOT NULL,
enabled_groups longtext NOT NULL,
disabled_mobile_groups longtext NOT NULL,
enabled_mobile_groups longtext NOT NULL,
post_priority int(3) NOT NULL default 0,
dir_count int(3) NOT NULL default 0,
user_role varchar(255) NOT NULL default ‘_’,