Question Problems after site upgrade to Joomla 4

Please go to the database and check if the collation is actually utf8_general_ci, or just displayed incorrectly in your configuration report.
Joomla 4.x and also Kunena 6.0 the collation should be utf8mb4.
If this is not the case, you must change the collation to utf8mb4.
You can do this either with phpMyAdmin or with the Admin Tools from Akeeba. With Admin Tools you just have to select UTF-8 Multibyte under "Change Database Collation" (it is already preset) and run it. After that (as recommended) click on "Repair & Optimise Tables".
But first create a database backup.

 Joomla 4.x and also Kunena 6.0 the collation should be utf8mb4.
 

Collation checked, it's the right one.
But I noticed this: trying to add a category it throws the error "1054 Unknown column 'parentid' in 'where clause'", but in the Categories table the column name is parent_id

Correct is parentid. Please create first a database backup, then rename this column to parentid. Afterwards test your page again.

 Correct is parentid. Please create first a database backup, then rename this column to parentid. Afterwards test your page again.

Done, much better now! Now I can see and use categories and discussions, but there is something else to adjust:

1. the root category is not shown
2. any attempt to modify any category, including the root one, ends in error: 0 3 arguments are required, 2 given

Is there a more detailed error message with debug mode?

Is there a more detailed error message with debug mode?

     0 3 arguments are required, 2 given

Call stack
1     ()     JROOT/libraries/src/Language/:283
2     sprintf()     JROOT/libraries/src/Language/:283
3     Joomla\CMS\Language\Text::sprintf()     JROOT/administrator/components/com_kunena/src/Controller/:186
4     Kunena\Forum\Administrator\Controller\CategoryController->internalSave()     JROOT/administrator/components/com_kunena/src/Controller/:252
5     Kunena\Forum\Administrator\Controller\CategoryController->apply()     JROOT/libraries/kunena/src/Controller/:319
6     Kunena\Forum\Libraries\Controller\KunenaController->executeTask()     JROOT/libraries/kunena/src/Controller/:226
7     Kunena\Forum\Libraries\Controller\KunenaController->execute()     JROOT/libraries/src/Dispatcher/:143
8     Joomla\CMS\Dispatcher\ComponentDispatcher->dispatch()     JROOT/libraries/src/Component/:355
9     Joomla\CMS\Component\ComponentHelper::renderComponent()     JROOT/libraries/src/Application/:143
10     Joomla\CMS\Application\AdministratorApplication->dispatch()     JROOT/libraries/src/Application/:186
11     Joomla\CMS\Application\AdministratorApplication->doExecute()     JROOT/libraries/src/Application/:294
12     Joomla\CMS\Application\CMSApplication->execute()     JROOT/administrator/includes/:61
13     require_once()     JROOT/administrator/:32