Thanks.
I made a backup of the old file. Then downloaded the new one, unpacked it and put it in the place of the old one.
I looked at the error logs again this morning.
Now I get these errors:
Backend log: PHP Warning: Attempt to read property "image_intro" on null in xxxpublic_html/plugins/system/t4/themes/base/html/com_content/article/default.php on line 35
Backend log: PHP Warning: Undefined variable $row in xxx/public_html/plugins/system/t4/themes/base/html/com_content/featured/default.php on line 70
Can this also be solved?