thank you for this quick and very complete answer.
I have uploaded ShowKase again, and it's OK now.

I use 2 Showkase on two providers. The one for a personal web, it's OK.
I purchase a second Showcase for an association with another provider.
sktest is OK (I need for this to create a directory "sessions" in root place).
When I start Showkase, I have the first page : "Welcome to your New Showcase Web Site... "
When I clic on Showcase Admin I get a blank page, in Safari and Firefox, on iMac.
What can I do ?
Cordially