I have trouble installing grav in a subdirectory of my server.

I have a static website running on the root level and several test projects running inside a /dev subfolder (static sites as other flat file cms) and one other site running in /my-other-project which also has its own URL.

However, if I go to www.mysite.com/dev/projectname/01 I get the following message in the browser:

The script could not be executed correctly.
Please refer to your error log for details about this error. You find it in your KIS under the item "Administration > YOUR PRODUCT > YOUR PACKAGE > Logfiles". Further information can also be found in our FAQ.

The logfile looks like this (added {…} for privacy):

{…} [php7:error] {…} {…} [host www.my-other-domain.com] script '/is/htdocs/…/www/my-other-domain.com/wp-login.php' not found or unable to stat
{…} [php7:error] {…} {…} [host www.my-other-domain.com] script '/is/htdocs/…/www/my-other-domain.com/wp-login.php' not found or unable to stat
{…} [core:alert] {…} {…} [host www.my-other-domain.com] /is/htdocs/…/www/dev/cp/01/.htaccess: Options not allowed here

The .htaccess seems to be read, because if I remove it I get the "Internal Server Error" message shown in the doc: https://learn.getgrav.org/16/troubleshooting/htaccess.

What am I getting wrong here?
Can I have a .htaccess file in the grav root or should I only have one in my main root folder?
If yes, how do I allow options?

Thanks in advance!

No in-depth knowledge here, but I've got multiple Grav installs running in multiple subdirectories and in each one I've got the default .htaccess file.

Have you checked file and folder permissions?

Some good info at the Grav Documents about permissions.
https://learn.getgrav.org/16/troubleshooting/permissions

Thanks for pointing that out, but I couldn't find any hint in there. I think, it's more about the .htaccess file.

I noticed that it's saying in the default .htaccess

## Begin RewriteBase
# If you are getting 500 or 404 errors on subpages, you may have to uncomment the RewriteBase entry
# You should change the '/' to your appropriate subfolder. For example if you have
# your Grav install at the root of your site '/' should work, else it might be something
# along the lines of: RewriteBase /<your_sub_folder>
##

# RewriteBase /

## End - RewriteBase

So I thought changing

# RewriteBase /

to

RewriteBase /dev/projectname/01/

would help, but it doesn't.