undefined index #conjunction

@ emptyvoid et al.: Did you try re-configuring your search view, as described in #16?

Rerolled the #19 patch for v8.x-1.31

Ha ha...This time the missing file is attached.

#16 fixed it for me

I'm getting this error using version 1.37. Since none of the suggestions about changing field types apply, I'm looking at the $query in the search method on search_api/modules/search_api_db/src/Plugin/search_api/backend/Database.php:

$query->__toString() = (string [402]) `Index: camp
Keys: NULL
Conditions:
  [
    field_specialt_ies IN array (
          1684 => '1684',
        )
  AND
    type IN array (
          'coach' => 'coach',
        )
  ]
Options: array (
    'search_api_view' => 'object (Drupal\\views\\ViewExecutable)',
    'search_api_retrieved_field_values' => 
    array (
    ),
    'skip result count' => false,
    'offset' => 0,
    'limit' => 12,

Nothing here looks obviously wrong to me.

@stephenplatz: Does the patch in #25 work for you?
Otherwise, what error and stack trace do you get?