I use Zen Cart for almost 10 years, never posted questions, because always found a solution reading this forum.
But now I am stuck

I have upgraded to 157 about a week ago from 155f via zc_install. Also changed 7.1 PHP to 7.4.9 PHP on server.
Using plugins Ultimate Seo Url, multilangual EZ-pages, payment module by Finnish payment service Bambora (can be found here: https://github.com/bambora/payform-zencart).
My site is here https://www.tulihelmi.fi/

But my problem was presented also before an upgrade with 155f and PHP 7.1.


Sometimes my customers have problem with payment using Bambora. There were single cases last autumn and January. Then in March and April more cases. Then in June and July few. Now in August it got worst and last days (also before an update) like every second order

Path goes like this:
- they push Confirm Order Button
- go to Bambora's page and pay
- redirecting back to shop

Normally must go to checkout_success page and in some cases it happens.
But in some cases they go to session expired page, so they got logged out.
In both cases I get paid and order registers into webshop, but customers are confused.

Also this happens to the same customer, who sometimes previously made an order without any problem. And next order same customer places withot problem. Seems to be random.
I also checked it and some times I can get back to checkout_success, sometimes get logged out.


I read forum. Seem to be the problem is that session cookies are lost is some cases.
Tried to change settings in admin/configurarions/sessions as advised. No difference.

Now using these settings again, as I always had:

Session directory /home/beadzsto/public_html/tulihelmi.fi/cache
Cookie Domain True
Force Cookie Use False
Check SSL Session ID False
Check User Agent False
Check IP Address True
Prevent Spider Sessions True
Recreate Session True
IP to Host Conversion Status true
Use root path for cookie path False
Add period prefix to cookie domain True

Both configure.php files have correct setting to my mind. Tried to change SQL_CACHE_METHOD to database, no difference.
PHP Code:
 * dist-configure.php - SAMPLE FILE!
 * @package Configuration Settings
 * @copyright Copyright 2003-2016 Zen Cart Development Team
 * @copyright Portions Copyright 2003 osCommerce
 * @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
 * @version $Id: Author: DrByte  Thu Dec 17 11:49:31 2015 -0500 Modified in v1.5.5 $
 * @private

/*************** NOTE: This file is VERY similar to, but DIFFERENT from the "admin" version of configure.php. ***********/
/***************       The 2 files should be kept separate and not used to overwrite each other.              ***********/

 * Enter the domain for your store
 * HTTP_SERVER is your Main webserver: eg-http://www.yourdomain.com
 * HTTPS_SERVER is your Secure/SSL webserver: eg-https://www.yourdomain.com

 *  If you want to tell Zen Cart to use your HTTPS URL on sensitive pages like login and checkout, set this to 'true'. Otherwise 'false'. (Keep the quotes)

 * These DIR_WS_xxxx values refer to the name of any subdirectory in which your store is located.
 * These values get added to the HTTP_CATALOG_SERVER and HTTPS_CATALOG_SERVER values to form the complete URLs to your storefront.
 * They should always start and end with a slash ... ie: '/' or '/foldername/'

 * This is the complete physical path to your store's files.  eg: /var/www/vhost/accountname/public_html/store/
 * Should have a closing / on it.

 * The following settings define your database connection.
 * These must be the SAME as you're using in your admin copy of configure.php
define('DB_TYPE''mysql'); // always 'mysql'
define('DB_PREFIX'''); // prefix for database table names -- preferred to be left empty
define('DB_CHARSET''utf8'); // 'utf8' or 'latin1' are most common
define('DB_SERVER''localhost');  // address of your db server

 * This is an advanced setting to determine whether you want to cache SQL queries.
 * Options are 'none' (which is the default) and 'file' and 'database'.

 * Reserved for future use
define('SESSION_STORAGE''temporary value added by zc_install');

 * Advanced use only:
 * The following are OPTIONAL, and should NOT be set unless you intend to change their normal use. Most sites will leave these untouched.
 * To use them, uncomment AND add a proper defined value to them.
// define('DIR_FS_SQL_CACHE' ...
// define('DIR_FS_DOWNLOAD' ...
// define('DIR_FS_LOGS' ...

// End Of File
I also was trying to surf through the store for some times checking if I get logged out, did not happen. But may be need to be in the shop longer, not sure.

Did not try any other payment module yet since the problem started, cos we have not very big choice of payment plugins made for Zen Cart in Finland.

So can it be a Bambora's problem or something wrong with my site? Or the server? A contacted my provider, they say they have nothing wrong.
Seems to be random sometimes working perfectly, sometimes not on a same user, browser and device.

Please, advice what else can I do
I love Zen Cart cos it is customisable... But this issue with payment ruining it all.