Cannot create subdirectory websites of secondary networks

Hi,

I have a multi-network installation. When I create a website (subdirectory style) within the primary network, everything works fine but when I create it in another network I get a 404 page (actully it is redirected to the default website of this network, as if it is seen as a permalink)

to be concrete :
https://maindomain.net => my primary network
https://maindomain.net/site1 => ok
https://maindomain.net/site2 => ok

https://domain2.net => another network, works well
https://domain2.net/site3 => redirected to https://domain2.net, with error 404
https://domain2.net/site4 => redirected to https://domain2.net, with error 404

any clue ?
thanks, best