Hi,
A fantastic plugin. I've just updated from (I think) a version 1.1 of the plugin, to the latest 1.32.90 - along with updating to WP 5.5 
Unsurprisingly it was asking to rebuild the index, so I clicked the rebuilt button, but it never completed.
Doing a bit of detective work, it seems to be, where it drops all the tables, then recreates them I'm getting the following:
[12-Aug-2020 13:38:08 UTC] WordPress database error Specified key was too long; max key length is 767 bytes for query CREATE TABLE wpftsi_index (
id int(10) unsigned NOT NULL auto_increment,
tid bigint(10) unsigned NOT NULL,
tsrc varchar(255) NOT NULL,
tdt datetime NOT NULL default '1970-01-01 00:00:00',
build_time int(11) NOT NULL default '0',
update_dt datetime NOT NULL default '1970-01-01 00:00:00',
force_rebuild tinyint(4) NOT NULL default '0',
locked_dt datetime NOT NULL default '1970-01-01 00:00:00',
PRIMARY KEY (id),
UNIQUE KEY tid_tsrc_unique (tid,tsrc),
KEY tid (tid),
KEY build_time (build_time),
KEY force_rebuild (force_rebuild),
KEY locked_dt (locked_dt),
KEY tsrc (tsrc)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci made by do_action('wp_ajax_wpftsi_submit_rebuild'), WP_Hook->do_action, WP_Hook->apply_filters, WPFTS_Core->ajax_submit_rebuild, WPFTS_Index->create_db_tables
select VERSION();
returns:
10.1.45-MariaDB-0+deb9u1
So for now I'm stuck. I think the problem, is utf8mb4 doesn't seem to allow an index longer than 191 characters when using utf8mb4 
https://make.wordpress.org/core/2015/04/02/the-utf8mb4-upgrade/
Talks about making sure indexes are the less that 191 long. Is there a way out of this, other than going back to the old version?!
Many thanks.
Rupert
