Upgrading Drupal

Hi,
today my Drupal 7 installation started not generating anymore the session cookie for anonimous users.

The problem is that I have some functionality, expecially during registration, thats works with the $_SESSION data.
I checked everyting and backtraced every function.

I dont know why but the session is started but the cookie not stored, so at every page load I lose the $_SESSION data.
How can I fix this or at list find where the problem is generated?

Thank you

I've not done a lot with migrations, the last major site rebuild was a D5->D7 and the migration was all done in SQL as there was no viable migrate process at the time, however, this was mostly 1 to 1 mappings so not a massively complex process.

Current scenario

Source is D6, target will be D8 (either 8.6 or 8.7 depending on project timescales)

I mad a new version of my site "the composer way" (https://www.drupal.org/forum/support/installing-drupal/2018-11-26/setting-up-site-if-you-use-the-composer-method#comment-12870385), installed all the modules that were on the old site (using composer), enabled them in the GUI, copied my files directory to the new site. Then I used the migrate GUI to migrate to D8.6.3.

But, in the new site, I have several issues: 

As per subject, i am going to upgrade our server's php version into 7.2. And i am using Drupal 7.60 (php 5.6.32) now, which i know that it will work fine with php 7.2.

My concern is more on the modules in it, thus the question here is whether anyone is aware if there is any ways/module/trick to check the compatibility of each modules with the php version 7.2 that i am going to install?

Any help will be appreciated!

Regards,

Frans

I have been having problems with all the versions of drush, composer, php, mysql and Ubuntu Server versions. I'm staying with Ubuntu Server 16.04 because there are still too many problems with the big changes in 18.04 Bionic Beaver. Then there are the incompatibilities with multiple versions of PHP 5.5, 5.6, 7.0, 7.1 and now 7.2 working with drush and composer.