I have the following warning in the admin panel:
No memory cache has been configured. To enhance your performance please configure a memcache if available.
I’ve read the following guide:
https://docs.nextcloud.com/server/12/admin_manual/configuration_server/caching_configuration.html
But I have no idea how to install the APCu. In the documentation on https://pecl.php.net/package/APCu, doesn’t help me much further.
Can anyone help me with this?
Which OS and php version are you using? (Debian, Ubuntu, CentOs,… | php5, php7.0,php7.1)
That’ll get you both APCu and Redis after running:
sudo apt install php-apcu redis-server php-redis on Ubuntu or equivalent on another distro. 
Is it better use both acpu and redis? I just installed/configured only redis and the message disappeared so I guesses it is enough?
Entirely up to you. Redis will do both if you like though is usually one of those things you turn to as your install scales. APCu is great otherwise normally.
I looked on your link, but it doesn’t help me much. I don’t know how to get the black prompt screens, where they put some code in.
I think, I can only browse the files on the server. And I have access to a cPanel?
This is why we have the support template, and why you should fill it out. Someone familiar with cPanel or Hostiso may be around, but it also may be faster to ask your host for assistance - you can link them here or to the guide above so they know exactly what you need.
I asked the Hostiso if they could fix the warnings. The replyed:
Hi, you can ignore it, we use diffrent setup.
Can I really ignore the warnings?
Can they tell you what they do? Ultimately it’s their system so you don’t have much leverage, but knowing would be nice.
Ok, I’ll ask.
This is what they answered:
It is opcache but not what NC recommend, that is to restrictive.
Are they right?