The problem with the respository was with the URL of /acs/admin/install ...:
https://www.misite.com/acs-admin/install/install?repository_url=http%3a%2f%2fopenacs.org%2frepository%2f5-1%2f&upgrade_p=1
This gives an Server Error because there is no www.openacs.org/repository/5-1/. I had to change 5-1 by 5-2
So the right URL is:
https://www.misite.com/acs-admin/install/install?repository_url=http%3a%2f%2fopenacs.org%2frepository%2f5-2%2f&upgrade_p=1
And it works
Cheers