Welcome to AbanteCart Ron!

Most common issues are related to session or permission on folders.
I was trying to check your site, but there is a redirect to some other place.

Is this correct?
www.multus-sdr.com/abantecart/

You can only do update of your files from upgrade package manually.

Since you see version changed I think SQL upgrade part was completed.

Check the error log under system/logs directory to see the error.

This depend on your PHP skills.
You can start looking into Task CLI script that is processing tasks.

https://github.com/abantecart/abantecart-src/blob/master/public_html/task_cli.php

You can create your own script to send desired emails or build a task controller & model that will do what you need.

Depending on the method that you choose, we can guide you more

We can help you. PM for discussion.

Check if the cache is working on your site and can be cleaned. Check directory permission.

Let me explain the root of this issue:

Then we sent a request to PayPal, and response does not tell us that transaction is finished within 5-10 seconds, we will show the message and let customer continue. We mark this order for manual verification.

There is a delay setting in the code that can be increased
public_html/extensions/default_pp_standart/storefront/view/default/template/responses/pending_ipn.tpl:31

Additionally in upcoming 1.2.13 there will be slight improvement.

Bottom line, I would not rely on PayPal Standard approach.

[SOLVED]
For those who have a similar issue, try removing certain extensions and check if they are the issue.
The issue was caused by an extension I had and has now been resolved as the developer's made a fix for it.
Thanks to all who helped.

Thank you for sharing. Was this the extension you did or something from the marketplace?

Glad you hear that you are happy with AbanteCart

We will schedule some time for improvement to notification system.