try to install a quick and clean install of a slave.

master: www.bijnlopvakantie.nl
slave: karinthie.bijnlopvakantie.nl

but i still receive a 403 error when opening the slave site. Please help.

When I go on the slave URL, it says that the access is forbidden.

Have you checked that you have correctly defined this subdomain in you httpserver.
You can test is with the "hello world" procedure.
Just copy the hello.php and hello.txt into the subdomain document root directory.
This will help you first check that you have correctly configured your http server and that you have the correct and expected document root directory.
See tutorial video 4 to 7 for the "hello word".
www.jms2win.com/tutorial#jms12x-7

If you have the symbolic link allowed, this mean that you can use the JMS deploy directory that must have the same value than the one you have defined for your subdomain.
When the website is created in JMS, you can check that you have all the directories and content deployed in the subdomain directory.

See the tutorial video CP-04 in case of a website deployed in a specific directory.
www.jms2win.com/tutorial#jms12x-CP-04