Problems after Update to core 8.35

Hi there.

I had problems after the update to core version8.35 - the backend was perfect, but no css was loaded, nor any other files, only the html.
reports said something about
Error: [] operator not supported for strings in Drupal\shortcode\Plugin\ShortcodeBase->addClass() (line 195 of /pages/0d/59/d0007358/home/htdocs/cms-8_benz-abc2017-neu/modules/shortcode/src/Plugin/ShortcodeBase.php)
I found a buggy solution to change the script from $classes [ ] =    to    $classes =
(simply erasing the not supported operator)

This fixed the problem, so now the homepage can be seen properly, but now the masonry portfolio doesn't show any links on hover anymore.
How can this be fixed?

EDIT:
I changed the core version to 8.28, but still, the masonry portfolio shows the picture, but is not clickable anymore. It doesn't even show the links, which should show on hover. As well as the view for the team doesn't show any team content anymore.