Did my test (i deleted the topic) triggered any error?

It seems it happens when a new topic title is searched for suggestions.

For example:

AAAAAAA BBBBB KK English CCCCC ZZZ DDDDDDD 1.1.1 (NNNNNNNNNNNNN release)

Here is a full debug log, again today

[08-Feb-2023 14:36:04 UTC] WordPress database error Regex error 'missing closing parenthesis at offset 11' for query SELECT `topicid` FROM wp_wpforo_topics WHERE `title` REGEXP "Nice|(live>" made by do_action('wp_ajax_wpforo_search_existed_topics'), WP_Hook->do_action, WP_Hook->apply_filters, wpforo\classes\Actions->search_existed_topics, wpforo\classes\Topics->search

[08-Feb-2023 14:36:03 UTC] WordPress database error Regex error 'missing closing parenthesis at offset 10' for query SELECT `topicid` FROM wp_wpforo_topics WHERE `title` REGEXP "Nice|(live" made by do_action('wp_ajax_wpforo_search_existed_topics'), WP_Hook->do_action, WP_Hook->apply_filters, wpforo\classes\Actions->search_existed_topics, wpforo\classes\Topics->search

[08-Feb-2023 14:36:03 UTC] WordPress database error Regex error 'missing closing parenthesis at offset 8' for query SELECT `topicid` FROM wp_wpforo_topics WHERE `title` REGEXP "Nice|(li" made by do_action('wp_ajax_wpforo_search_existed_topics'), WP_Hook->do_action, WP_Hook->apply_filters, wpforo\classes\Actions->search_existed_topics, wpforo\classes\Topics->search

Example of topic title triggering the above starts with:

Nice (live?) xxxxxxxxxxxxxxxxxxxxxx

I would guess something is wrong with parenthesis, since both topics contained some "(xxxxx)"

I can replicate this. Just start a Topic and write the title with

"Nice (live?) something something"

Paste the title doesn't trigger this, TYPING it letter by letter does that.

I hope i helped.