Dear Ciaran,
I don't why, but apparently joomla 4 wasn't installed completely.
So after your message, I looked again at the Joomla! 4.x Install Files, which I got through unzipping.
I unzipped in this case the j51_stockholm_j4 file as well and found out that not only the error.php was in it, and misteriously not installed while upgrading to joomla 4 by uploading the zipped the j51_stockholm_j4 file
Also other files were missing, like less and cscc.
Strange, since the upgrade went well.
But I uploaded all the missing files through filezilla, one by one.
The problem of the missing files is therefore now solved.

Hi Annechien

You should find that there is already an error.php in place on your template (../templates/j51_stockholm/error.php)?

Ciaran

Dear Ciaran

I know you are very busy. But do you know when you could add the missing error.php file to the Stockholm Template Joomla 4? Thank you in advance.
Have a nice sunday, Annechien

Dear Ciaran. That is of course where I searched. But the file is simply not there, see screenshot.
I therefore tried to import the error.php I used in the Joomla 3 version. But that does not work.
The error.php is also not located anywhere else. I searched every folder. It contains a error.css file but that is not the one I should adapt of course.

So that's why I requested you to update: to do the same as was apparently wrong in the Scarlet template as well, see
https://www.joomla51.com/forum/add-custom-404-page-to-scarlet-template

Would that be possible?
Thank you in advance
Best regards
Annechien

Hi Annechien

You should be able to find your error.php in the root of your template folder (../templates/j51_stockholm/error.php)?

Ciaran