It seems that oneclicklogin was the culprit

-sh-4.1$ grep -R _autoRegister *
plugins/payplansregistration/activateafterpayment/activateafterpayment.php: $userId = $this->_autoRegister($username, $email, $password);
plugins/payplansregistration/activateafterpayment/activateafterpayment.php: function _autoRegister($username, $email, $password)
plugins/payplansregistration/auto/auto.php: $userId = $this->_autoRegister($username, $email, $password,$fullname);
plugins/payplansregistration/auto/auto.php: function _autoRegister($username, $email, $password, $fullname=null)
plugins/payplans/oneclickcheckout/oneclickcheckout/invoice.php: $userId = $this->_autoRegister();
plugins/payplans/oneclickcheckout/oneclickcheckout/invoice.php: protected function _autoRegister()
plugins/payplans/oneclickcheckout/oneclickcheckout/invoice.php: $userId = XiHelperPlugin::getPluginInstance('payplansregistration', $registrationPlugin)->_autoRegister($username, $email, $password,$username); //fullname is equal to username.
plugins/payplans/oneclickcheckout/oneclickcheckout/invoice.php: ->_autoRegister(
plugins/payplans/donation/donation.php: ->_autoRegister(

I disabled it and the site came back up... Maybe some forensic work on your side to see the reason?

Hello Ahamd,

I checked your site. You are using Activate After payment plugin for registration, which is dropped in past payplans releases.
This functionality is added in Payplans Quick Registration plugin , so you need to use Quick registration . I have enabled quick registration and One click checkout, no error is shown related _autoRegister function.

But the issue is that When you use Oneclick checkout plugin , it will create a dummy user (Not_Registered) but on your site you have applied validation that username must be lower case, that's why it failed to create dummy user.
https://stackideas.com/docs/payplans/administrators/quick-solutions/concept-of-not-registered-user

To use oneclickcheckout plugin at your site , do the below mentioned changes.
Go to root/plugins/payplans/oneclickcheckout/oneclickcheckout/invoice.php, find function namegetDummyUserId
In this functiona change Not_Registered to not_registered.

Do the changes and enable oneclickcheckout plugin and update me if still issue occurs and also provide me site ftp details.
So i can check this issue.

Its okay... I dont really need oneclickcheckout... but I do need the EasySocial registration plugin which I have enabled... I couldnt find the activate after payment plugin that you are talking about..

I see another error though...

e.g go to https://zainabcenter.org/membership/enrollment/group/4

Click on any of the plans without logging in...e.g https://zainabcenter.org/component/payplans/plan/login/17?Itemid=329

It shows as FREE now... why the wrong information? Please advise as our registration season is about to start

Hello Ahmad,

We found the issue recently , it's coming due to advance pricing app. You have not setup advance pricing app, only plugin is enabled.
For now, to resolve this issue i've disabled advance pricing app.

We are working on this issue and will release this fix soon.

Hello Ahamad,

I've already applied fix on your site related to advance pricing app.
We will release this fix soon.

Thank you for understanding.