Subpage not reachable any more

I have a page "www.mydomain.com/sub" which is not reachable any more (Forbidden)

What I noticed:

1. When I request "www.mydomain.com/sub" in the browser it gets extended to www.mydomain.com/sub/ (with final slash). Who is adding that slash?
2. Reuploading the "sub" pages under "sub2" works.

What might have caused it:
I defined a subdomain at my webhoster "sub.mydomain.com" pointing to "/grav/sub" in my webspace

That looks like a .htaccess problem (if using Apache). If sub2 works and you have setup a subfolder rule for /sub, that might interfere.

The slash, same thing, looks like a htaccess rule. Grav does not add a slash.