500 internal error in each post

Hi,
I wish a multisite with follow configuration :

main network : http://main.domain.it
site 1 : http://sub1.domain.it
site 2 : http://sub2.domain.it
out of network : http://www.domain.it

when install multisite option, WP set this subdomain for site (I haven’t other choice):
main network : http://main.domain.it
site 1 : http://sub1.main.domain.it
site 2 : http://sub2.main.domain.it

Then I configured the server (with cpanel) with this subdomain and path:
* –> /www
main –> /main.domain.it
sub1 –> /main.domain.it
sub2 –> /main.domain.it
(using “parking” all is redirect to http://www.domain.it)

.htaccess is as wp write in network configuration, but :
RewriteBase /main.domain.it/
(else all is redirect to http://www.domain.it)

domain mapping :
sub1.domain.it –> site 1 (primary)
sub1.domain.it –> site 2 (primary)

in “all site” > edit > domain
site 1 : sub1.domain.it
site 2 : sub2.domain.it
(else site dashboard is unreachable)

now :
home page is ok for all sites
dashboard is reached for all sites
for all other pages I get “500 internal server error”

WP is clean (Hello word post), with Twenty Fifteen theme and only domain mapping plugin is installed.

What is wrong?

https://ww.wp.xz.cn/plugins/wordpress-mu-domain-mapping/