Community auth problems

Dear all,
to obtain NMI certification, we upgraded php from 5.6 to 7.2.

With php 5.6, 7.0 all work fine, with that new release we cannot login correctly to our platform.

Also, application wrote all datas into DB, as user logged in but i'm always redirected to home page.

I think it's probably a problem about sessions or cookies, any idea?

Thanks

Riccardo