500 error on wp-dashboard after login

Hi there,

I keep getting a 500 error on my admin dashboard.
Refreshing gives me: error/no-error after a while only error

Tried this:
– new htaccess file and save permalinks
– copy fresh wp-admin / wp-includes folders
– removed the ./ in this line: require(‘./wp-blog-header.php’);
– increased memory: memory=64MB
– activate/deactivate plugins
– asked the hostingpartner (only windows server / no Unix unfortunately)

WP Debug says:
Undefined offset: 2 in advanced textwidget ??