I switched the setting "Who has read this thread - Mode" from "last read date" to "all dates" and got the following error:
INSERT INTO `xfa_wrtt_thread_read` (`thread_id`, `user_id`, `thread_read_date`) VALUES (?, ?, ?)
------------
#0 src/XF/Db/Mysqli/Statement.php(196): XF\Db\AbstractStatement->getException('MySQL query err...', 1062, '23000')
#1 src/XF/Db/Mysqli/Statement.php(77): XF\Db\Mysqli\Statement->getException('MySQL query err...', 1062, '23000')
#2 src/XF/Db/AbstractAdapter.php(94): XF\Db\Mysqli\Statement->execute()
#3 src/XF/Db/AbstractAdapter.php(221): XF\Db\AbstractAdapter->query('INSERT INTO `x...', Array)
#4 src/addons/XFA/WhoReadThisThread/XF/Repository/Thread.php(72): XF\Db\AbstractAdapter->insert('xfa_wrtt_thread...', Array)
#5 src/XF/Pub/Controller/Thread.php(89): XFA\WhoReadThisThread\XF\Repository\Thread->markThreadReadByVisitor(Object(XFA\WhoReadThisThread\XF\Entity\Thread), 1597160619)
#6 src/XF/Mvc/Dispatcher.php(350): XF\Pub\Controller\Thread->actionIndex(Object(XF\Mvc\ParameterBag))
#7 src/XF/Mvc/Dispatcher.php(261): XF\Mvc\Dispatcher->dispatchClass('XF:Thread', 'Index', Object(XF\Mvc\RouteMatch), Object(XFA\WhoReadThisThread\XF\Pub\Controller\Thread), NULL)
#8 src/XF/Mvc/Dispatcher.php(113): XF\Mvc\Dispatcher->dispatchFromMatch(Object(XF\Mvc\RouteMatch), Object(XFA\WhoReadThisThread\XF\Pub\Controller\Thread), NULL)
#9 src/XF/Mvc/Dispatcher.php(55): XF\Mvc\Dispatcher->dispatchLoop(Object(XF\Mvc\RouteMatch))
#10 src/XF/App.php(2190): XF\Mvc\Dispatcher->run()
#11 src/XF.php(391): XF\App->run()
#12 index.php(20): XF::runApp('XF\\Pub\\App')
#13 {main}
- XF\Db\DuplicateKeyException: MySQL query error [1062]: Duplicate entry '3033-432684' for key 'user_id_thread_id'
- src/XF/Db/AbstractStatement.php:228
INSERT INTO `xfa_wrtt_thread_read` (`thread_id`, `user_id`, `thread_read_date`) VALUES (?, ?, ?)
------------
#0 src/XF/Db/Mysqli/Statement.php(196): XF\Db\AbstractStatement->getException('MySQL query err...', 1062, '23000')
#1 src/XF/Db/Mysqli/Statement.php(77): XF\Db\Mysqli\Statement->getException('MySQL query err...', 1062, '23000')
#2 src/XF/Db/AbstractAdapter.php(94): XF\Db\Mysqli\Statement->execute()
#3 src/XF/Db/AbstractAdapter.php(221): XF\Db\AbstractAdapter->query('INSERT INTO `x...', Array)
#4 src/addons/XFA/WhoReadThisThread/XF/Repository/Thread.php(72): XF\Db\AbstractAdapter->insert('xfa_wrtt_thread...', Array)
#5 src/XF/Pub/Controller/Thread.php(89): XFA\WhoReadThisThread\XF\Repository\Thread->markThreadReadByVisitor(Object(XFA\WhoReadThisThread\XF\Entity\Thread), 1597160619)
#6 src/XF/Mvc/Dispatcher.php(350): XF\Pub\Controller\Thread->actionIndex(Object(XF\Mvc\ParameterBag))
#7 src/XF/Mvc/Dispatcher.php(261): XF\Mvc\Dispatcher->dispatchClass('XF:Thread', 'Index', Object(XF\Mvc\RouteMatch), Object(XFA\WhoReadThisThread\XF\Pub\Controller\Thread), NULL)
#8 src/XF/Mvc/Dispatcher.php(113): XF\Mvc\Dispatcher->dispatchFromMatch(Object(XF\Mvc\RouteMatch), Object(XFA\WhoReadThisThread\XF\Pub\Controller\Thread), NULL)
#9 src/XF/Mvc/Dispatcher.php(55): XF\Mvc\Dispatcher->dispatchLoop(Object(XF\Mvc\RouteMatch))
#10 src/XF/App.php(2190): XF\Mvc\Dispatcher->run()
#11 src/XF.php(391): XF\App->run()
#12 index.php(20): XF::runApp('XF\\Pub\\App')
#13 {main}