Here is how I fixed it:
To gain access to your widget page, simply comment out two lines of your wp-config.php file:
// define('DB_CHARSET', 'utf8');
// define('DB_COLLATE', '');
Then log in and go to your widget page.
Remove each text widget and PHP widget (copy the content into a text editor so you can retrieve it afterwards again).
Go back into your wp-config.php file and remove the comments again. Save.
Log in and add your text widgets again. This should work fine now.
