Upgrade from 4.8.2 to 5.1 breaks my site

I have an Ivory Search form that searches all pages, posts, and media for all search words (and AND search) and displays 5 search results per page. When my site used Ivory Search 4.8.2 the search for the one word “baatz” (without the quotes) returns 23 results, some pages, some posts, and some media (images/photos). My site automatically updated to Ivory Search 5.1 and the same search now returns the below in my debug.log. I presume that the most interesting part is the “illegal argument to a regular expression,” but I’ll leave that determination to you. Do you need further information from me to determine the problem and the fix or whether I am doing something wrong?

[22-Dec-2021 14:56:58 UTC] [warning]: 'mysqli_query(): (HY000/3685): Illegal argument to a regular expression.' file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-includes/wp-db.php, line: 2056
[22-Dec-2021 14:56:58 UTC] **************************** Trace start ****************************
  [0]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-content/plugins/add-search-to-menu/includes/class-is-debug.php; line: 98; function: debug_trace; class: IS_Debug
  [1]function: process_error_backtrace; class: IS_Debug
  [2]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-includes/wp-db.php; line: 2056; function: mysqli_query
  [3]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-includes/wp-db.php; line: 1945; function: _do_query; class: wpdb
  [4]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-includes/wp-db.php; line: 2657; function: query; class: wpdb
  [5]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-includes/class-wp-query.php; line: 3024; function: get_col; class: wpdb
  [6]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-includes/class-wp-query.php; line: 3465; function: get_posts; class: WP_Query
  [7]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-includes/class-wp.php; line: 629; function: query; class: WP_Query
  [8]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-includes/class-wp.php; line: 752; function: query_posts; class: WP
  [9]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-includes/functions.php; line: 1291; function: main; class: WP
  [10]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-blog-header.php; line: 16; function: wp
  [11]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/index.php; line: 17; function: require
[22-Dec-2021 14:56:58 UTC] WordPress database error Illegal argument to a regular expression. for query SELECT SQL_CALC_FOUND_ROWS DISTINCT wp_wnmnr9_posts.ID FROM wp_wnmnr9_posts  WHERE 1=1  AND (  ((wp_wnmnr9_posts.post_title REGEXP '([[:<:]])baatz|baatz([[:>:]])') OR (wp_wnmnr9_posts.post_content REGEXP '([[:<:]])baatz|baatz([[:>:]])') OR (wp_wnmnr9_posts.post_excerpt REGEXP '([[:<:]])baatz|baatz([[:>:]])'))) AND wp_wnmnr9_posts.post_type IN ('post', 'page', 'attachment') AND ((wp_wnmnr9_posts.post_status = 'publish' OR wp_wnmnr9_posts.post_status = 'inherit'))  ORDER BY wp_wnmnr9_posts.post_title LIKE '%baatz%' DESC, wp_wnmnr9_posts.post_date DESC LIMIT 0, 5 made by require('wp-blog-header.php'), wp, WP->main, WP->query_posts, WP_Query->query, WP_Query->get_posts
[22-Dec-2021 14:56:58 UTC] [warning]: 'Trying to get property 'post_type' of non-object' file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-content/themes/boldgrid-gridone/base.php, line: 15
[22-Dec-2021 14:56:58 UTC] **************************** Trace start ****************************
  [0]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-content/plugins/add-search-to-menu/includes/class-is-debug.php; line: 98; function: debug_trace; class: IS_Debug
  [1]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-content/themes/boldgrid-gridone/base.php; line: 15; function: process_error_backtrace; class: IS_Debug
  [2]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-includes/template-loader.php; line: 106; function: include
  [3]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-blog-header.php; line: 19; function: require_once
  [4]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/index.php; line: 17; function: require
[22-Dec-2021 14:56:58 UTC] [warning]: 'Trying to get property 'post_type' of non-object' file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-content/themes/boldgrid-gridone/base.php, line: 16
[22-Dec-2021 14:56:58 UTC] **************************** Trace start ****************************
  [0]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-content/plugins/add-search-to-menu/includes/class-is-debug.php; line: 98; function: debug_trace; class: IS_Debug
  [1]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-content/themes/boldgrid-gridone/base.php; line: 16; function: process_error_backtrace; class: IS_Debug
  [2]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-includes/template-loader.php; line: 106; function: include
  [3]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/wp-blog-header.php; line: 19; function: require_once
  [4]file: /home/dh_rxzmb3/eribaa1.dreamhosters.com/index.php; line: 17; function: require

Eric Baatz