500 internal server error

If you have ssh access that would be better.
The just move the /wp-content folder out of your root, and use the clone tool in phpMyAdmin so you can duplicate the db.
https://www.bluehost.com/help/article/how-to-copy-a-database-using-phpmyadmin

Or, even without ssh – you can still copy the existing /wp-content folder outside the root.

This approach, you don’t need to download anything locally, and you still have what you need backed up.

Now you have something conrete to send to your host support!
??

Learning so many new things today, thank you ??

Interestingly, cloning the database is also not allowed:

CREATE DATABASE IF NOT EXISTS p490226_nl12k DEFAULT CHARSET=utf8 COLLATE utf8_general_ci;

MySQL meldt:

#1044 – Toegang geweigerd voor gebruiker: ‘p490226_nl21h’@’%.dbp.shared.prod.hostnetbv.nl’ naar database ‘p490226_nl12k’

I’ll send that to the host as well.

Groan.
Time for a new host – this is basic stuff that you should be able to do yourself.

Did you try to clone using the Operations tools under phpMyAdmin?
https://content.bluehost.com/bluehost/img/bluehost/cpanel/phpmyadmin/phpmyadmin-copy-database.png

Yes, I tried it that way. This error occurs:

Warning in ./libraries/classes/Dbal/DbiMysqli.php#244
mysqli::query(): MySQL server has gone away

Unfortunately my client just chose this new host (I had no say in it), we moved there a few weeks ago.

Is there anyone else who can help?

imo this falls under hosting support – I’m not sure there is any more advice to help that has not already been provided.
If ths host support is not good, you’ll need to go through your server / php settings to increase the timeouts to see if it helps:
https://kinsta.com/knowledgebase/mysql-server-has-gone-away/
https://www.fixrunner.com/mysql-server-has-gone-away-error/
https://www.linode.com/community/questions/18617/how-do-i-fix-the-mysql-server-has-gone-away-error

Or – migrate the site over to a staging site on a different server and see if you get the same results. Then you can narrow down if it is the site or the hosting.
You need to start eliminating the possibilties.