Strange error: Only mainsite database error, subsites working fine.

I just moved my multisite from one server to an other server. All the DNS settings are changed, and the sites are working fine. Expect the main site… There I get Error establishing a database connection.

I already tested the database connection with the PHP test file as WPbeginner suggested. The connection is working fine. All the subsites are up and running correctly. Only the mainsite gots the error.

I really have no clue at all what is happing here. How can the main site have a database error, while the others are working fine?

Thanks for the help!