Please Help, can’t access dashboard

Please help.

Here is what i did: I moved my to a new domain and host; did a backup of the old page’s database, dragged all the files to the new host, updated the config.php to acces the new database in which i imported the backup.
Ran the query:
UPDATE wp_options SET option_value = replace(option_value, ‘http://olddomain.com’, ‘http://www.newdomain.com’) WHERE option_name = ‘home’ OR option_name = ‘siteurl’;

My page is working fine, but appears blank when i try to acces the dashboard.
All the links inside my page work correctly, and all the pluggins also work correctly. I didn’t changed the wordpress version eather.

I deactivated the pluggins through the database and/or through FTP. With both methods the page remains blank and with an error message: “Warning: Cannot modify header information – headers already sent by (output started at /home/content/81/5229581/html/saps/wp-config.php:1) in/home/content/81/5229581/html/saps/wp-includes/pluggable.php on line 850”

I also placed the permissions to exactly how they are at the old page. but not a single one of the above changes a thing (nor in combination), I don’t know what else to do.

I would appreciate help. I certanly need it.
Sencirely, david.
Thank you in advance.