Question Error Class "JFactory" not found after upgrade to Kunena 7.0

After upgrading to Kunena 7.0, the Front-End of my websites is no longer available. The following error is being displayed:
0 - Class "JFactory" not found

The Back-End works as it should.
I am on Joomla 6.0.2. The compatibility plugin is enabled. 
I opened the Kunena Dashboard after the upgrade and a message confirmed, that the database tables were also updated.

The update process was very strange. 

Step 1
Kunena Forum update.kunena.org/6.3/pkg_kunena.xml      Site     Paket     Installed 6.3.10     Available 6.4.8
This update could not be installed. Error «Invalid extension update».

Step 2
I downloaded Kunena 7.0 and tried to install.
Error «You cannot not upgrade Kunena from version 6.3.10, you can only upgrade from version 6.4.5 or higher»

Step 3
I downloaded the latest Kunena pkg_kunena_v6.4.8_2025-12-06 and tried to install. 
Error (I do not remember the exact error message).

Step 4
I downloaded the Kunena pkg_kunena_v6.4.5_2025-08-23 and tried to install.
Success. 
I opened the Kunena Dashboard after the upgrade and a message confirmed, that the database tables were also updated.

Step 5
I downloaded Kunena 7.0 and installed it successfully.
I opened the Kunena Dashboard after the upgrade and a message confirmed, that the database tables were also updated.
When I opened the Front-End, the error message 0 - Class "JFactory" not found was displayed.
I cleared all caches but the error message in the Front-End remains. 
So I restored the previously created backup. 
My site is working now, but with a fallback to Kunena 6.3.10.
And again the update to Kunena 6.4.8 is being recommended:
 

Have you already tried Clear cache in Joomla management section?

Leen wrote:

Have you already tried Clear cache in Joomla management section?
 

Yes I did.

Trying to install the recommended update from 6.3.10 to 6.4.8. Error:

So I download Kunena 6.4.5 from github.com/Kunena/Kunena-Forum/releases/tag/6.4.5 and install.
Success.

But I will now certainly refrain from upgrading to Kunena 7!

Hello,
I now notice that you edited your first question slightly
after I asked if you had cleared the Joomla cache.
So now it's difficult to find a good answer, but why don't you test this locally first with a Wamp or Xamp server?