Advanced search in "message text only" crashes with SQL error when using Mysql fulltext search index

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