Error establishing a database connection

I changed LOGIN in My SQL, now I have problems.
1. I cannot log in to WORDPRESS.

2. On my website, some pages do connect en some don’t.

This works: https://www.spijkerenhamer[].]nl/

This works not: https://www.spijkerenhamer[.]nl/teamcoaching/
and gives the messge
“Warning: Cannot modify header information – headers already sent by (output started at /home/marliekeo.nl/public_html/spijkerenhamer.nl/index.php:4) in /home/marliekeo.nl/public_html/spijkerenhamer.nl/wp-includes/functions.php on line 3634
Error establishing a database connection”

• This topic was modified 6 years, 9 months ago by spijkb.
• This topic was modified 6 years, 9 months ago by Steven Stern (sterndata).

The page I need help with: [log in to see the link]