PHP error on posting a response

Using latest Kunena & Joomla 3.9.xx, each time a user posts a response (including me), we get the error message below:

Hi knro,

Please let us know the error message on your website. We will check and help you to solve the issue.

Best regards,

I tried to post a screenshot of the error but it wouldn't let me. I'll paste the text

Call stack
# Function Location
1 () JROOT/libraries/joomla/database/driver/mysqli.php:254
2 JDatabaseDriverMysqli->escape() JROOT/libraries/joomla/database/driver.php:1871
3 JDatabaseDriver->quote() JROOT/components/com_community/libraries/core.php:178
4 CFactory::getUser() JROOT/plugins/kunena/community/activity.php:180
5 KunenaActivityCommunity->onAfterReply() JROOT/libraries/kunena/integration/activity.php:85
6 KunenaIntegrationActivity->__call() JROOT/libraries/kunena/forum/message/message.php:992
7 KunenaForumMessage->update() JROOT/libraries/kunena/forum/message/message.php:740
8 KunenaForumMessage->save() JROOT/components/com_kunena/controllers/topic.php:754
9 KunenaControllerTopic->post() JROOT/libraries/kunena/controller.php:324
10 KunenaController->executeTask() JROOT/libraries/kunena/controller.php:215
11 KunenaController->execute() JROOT/components/com_kunena/kunena.php:114
12 require_once() JROOT/libraries/src/Component/ComponentHelper.php:402
13 Joomla\CMS\Component\ComponentHelper::executeComponent() JROOT/libraries/src/Component/ComponentHelper.php:377
14 Joomla\CMS\Component\ComponentHelper::renderComponent() JROOT/libraries/src/Application/SiteApplication.php:194
15 Joomla\CMS\Application\SiteApplication->dispatch() JROOT/libraries/src/Application/SiteApplication.php:233
16 Joomla\CMS\Application\SiteApplication->doExecute() JROOT/libraries/src/Application/CMSApplication.php:225
17 Joomla\CMS\Application\CMSApplication->execute() JROOT/index.php:49

Hi,
This isn't error of the template. Please provide us the your website url (You can send us to mail box). We will try to help you find the reason of error.

Best regards,