Since version 1.7 of PrestaShop, the Cookie SameSite feature causes a session loss upon return when POST mode is enabled.

2 solutions exist to solve your problem:

Change return mode to GET

To change the return mode :

1. Sign in to your Back Office.
2. Go to the Modules menu > Module Manager.
3. Search for the Scellius module.
4. Click on Configure.
5. Click on General configuration.
6. Search for Return to shop.
7. Select GET in the Return mode parameter.
8. Click Save.

Disable the Cookie SameSite

If you wish to keep the POST return mode, make the following change:

1. Sign in to your Back Office.
2. Go to Shop settings > General settings.
3. Set Enable SSL to Yes.
4. Set Enable SSL on the whole website to Yes.
5. Click Save.
6. Go to Advanced parameters > Administration.
7. Set Cookie SameSite to None.
8. Click Save.