Sry for bringing up this topic, but unfortunately the older / other answers did not help.
Usually I am running my dev-projects locally, pushing them to mysite.com/clientname later to show them to the client.
On the Server I created a subdirectory called clientname, where I put the all files.
I further updated the .htaccess to RewriteBase /clientname (this is everything I changed there)
In another thread I read you could (or should?) update the config.php as well, so I included c::set('url', 'http://www.mydomain.com/clientname');
I tried the .htaccess in the root directory as well as the /clientname directory.
But unfortunately I am just getting an HTTP ERROR 500.
I would normally suggest using subdomains (ie: clientname.mysite.com) for this, to prevent any settings / configurations leaking through. Is that an option for you? Just be sure you adhere to Kirby licensing terms.
I also had a look at the documentation where it reads (at least for panel 500 issues) one should have a look into php md string stuff. but this is just locally right?
What kind of hosting is it? Shared? VPS? Something else?.. it might be there are things missing and you may or not have the rights to install them, this is usually true of shared hosting.
If you log into your hosting control panel, there might be a method to pick the version of PHP you want to run. Since you are using a subfolder though, changing will effect the main site above it.
This is another reason for using subdomains, since the php version can (usually) be set independently.