When search method is set to 'fulltext index' the settings goes away and shows '[] operator not supported for strings' message only.
Affected code and bug fix:
Sources/ManageSearch.php
$context['fulltext_index'] = '';
if ($request !== false || $smcFunc['db_num_rows']($request) != 0)
{
Find and change to:
$context['fulltext_index'] = array();
if ($request !== false || $smcFunc['db_num_rows']($request) != 0)
{
It's a known issue:
https://www.simplemachines.org/community/index.php?topic=554723.0
Fixed in 2.0.16