Solved Kunena view 'administrator' not found

Hi guys,
after editing Joomla4 user.css file at the backend, i got this error "Kunena view 'administrator' not found" when I try to save the changes in that file.
Everything was fine till now. The only change I made to the website was editing some Menu items, but I can't figure out what this has to do with the "fatal error".

Here is a video to show what happens:
www.screentype.net/screen.mov
 

I don't know true reason but in right corner I saw Joomla 4.4.3.
Kunena 6.3 needs Joomla: version 4.4.7 or greater (>= 5.1.3 recommended)

It's very strange really, because everything was fine before.
I don't think it's the version that's causing the problem because, as I said, before today everything was fine.
It's melting my head...

Hi xamoix,
Try to do some below steps to check the issue:
- First, use a FPT program  to remove the text line you just added.
- If it works properly, update your website to the Joomla latest version 4.4.8.
- Next, try adding a comment in the form /* YOUR COMMENT HERE */. Save it to see if it has the same problem as above?

I've been analyzing all the changes I've made to the user.css in the last two days and I've found the cause of the problem, (but I don't know why it's happening).
The problem is this line of CSS that I added to user.css to use Sweetalert when displaying system messages: After removing the line, the user.css file can be edited again in the backoffice without any problem. I had to create another css file with just that line of CSS and place it in the template's css folder.
It's all very strange but the problem has been solved.