With Mysql 5.7 engine InnoDB and phpbb using mysql fulltext search index, any advanced search with a setting "Search within: Message text only" results in SQL general error:

[code] SQL ERROR [ mysqli ]

Can't find FULLTEXT index matching the column list [1191]

SQL

SELECT SQL_CALC_FOUND_ROWS p.post_id FROM phpbb_posts p WHERE MATCH (p.post_text) AGAINST ('+video ' IN BOOLEAN MODE) AND (p.post_visibility = 1 OR p.forum_id IN (1, 2, [long list of all forum IDs], 144)) ORDER BY p.post_time DESC LIMIT 250

BACKTRACE

FILE: (not given by php)
LINE: (not given by php)
CALL: msg_handler()

FILE: [ROOT]/phpbb/db/driver/driver.php
LINE: 855
CALL: trigger_error()

FILE: [ROOT]/phpbb/db/driver/mysqli.php
LINE: 193
CALL: phpbb\db\driver\driver->sql_error()

FILE: [ROOT]/phpbb/db/driver/mysql_base.php
LINE: 45
CALL: phpbb\db\driver\mysqli->sql_query()

FILE: [ROOT]/phpbb/db/driver/driver.php
LINE: 261
CALL: phpbb\db\driver\mysql_base->_sql_query_limit()

FILE: [ROOT]/phpbb/db/driver/factory.php
LINE: 321
CALL: phpbb\db\driver\driver->sql_query_limit()

FILE: [ROOT]/phpbb/search/fulltext_mysql.php
LINE: 573
CALL: phpbb\db\driver\factory->sql_query_limit()

FILE: [ROOT]/search.php
LINE: 590
CALL: phpbb\search\fulltext_mysql->keyword_search()
[/code]

https://www.phpbb.com/community/viewtopic.php?f=466&t=2421931