Blank Screen on Checkout - no messages w/debug on

-- HikaShop version -- : 3.0.0
-- Joomla version -- : 3.6.5
-- PHP version -- : 7.0.14
-- Browser(s) name and version -- : Firefox 51.0.1 64 bit
-- Error-message(debug-mod must be tuned on) -- : (blank screen)

We recently did a big upgrade from Joomla 2.5 running an older Hikashop version to the current of everything. Most of it has come together now, but I am having an issue with checkout. Here is the situation:

We are selling, among other things, Moodle eLearning classes using Joomdle-Hikashop integration. I am testing a checkout where the item is free, so there is no payment plugin involved. Once I hit the final submit button, the page "loads" for a long time (nothing changes on the screen). I receive three enrollment email messages from Moodle. These messages come nearly one minute apart and are identical, aside from the timestamp (and I should only be getting one). A few seconds after the third enrollment email, I finally receive the order creation email from Hikashop. Almost immediately after I get that email, the web page finally stops "loading" and goes blank.

I checked the forums and the first step seems to be to to turn on debugging, which I did, but the blank screen is totally blank. A "view source" is empty.

Any help would be appreciated!

Ok, did a couple of tests and got some error messages.

Test 1: I disabled all of the Joomdle plugins and I changed the Joomla SMTP config to point to a non-existent server. Joomla debug was On and Error reporting was set to Maximum. I also changed the php version from 7 to 5.6.

It did not take long for the page to load, it wasn't blank and I got this error message:
Warning: Creating default object from empty value in {path to site}/administrator/components/com_hikashop/classes/order.php on line 786

Test 2: I re-enabled the Joomdle plugins. This time it took a long time, but at least didn't go to a white page. I got a these messages:

Strict Standards: Only variables should be assigned by reference in {path to site}/plugins/system/joomdlehikashopstartdate/joomdlehikashopstartdate.php on line 38 (and this error again for line 47)

Warning: Invalid argument supplied for foreach() in {path to site}/plugins/system/joomdlehikashopstartdate/joomdlehikashopstartdate.php on line 105

Strict Standards: Only variables should be assigned by reference in {path to site}/plugins/system/joomdlehikashopgroups/joomdlehikashopgroups.php on line 38 (and this error again for line 47)

Warning: Invalid argument supplied for foreach() in {path to site}/plugins/system/joomdlehikashopgroups/joomdlehikashopgroups.php on line 105

Warning: Creating default object from empty value in {path to site}/administrator/components/com_hikashop/classes/order.php on line 786

Does any of this help? I've also attached a text file with these errors plus what was dumped to the server log.

Making some progress here. The multiple email message were the result of having too many enrollment plugins installed for Moodle. I disabled the ones that I didn't need and now I get just one enrollment email. It still takes a while to process the order and I am still getting this error:

Warning: Creating default object from empty value in {site_path}/administrator/components/com_hikashop/classes/order.php on line 786

Another bit of information: I still get the Warning (mentioned in my previous post) when I purchase items that are regular store items. So in that case Joomdle is not involved.

I hadn't tested this without Error Reporting. I just did and the white screen is gone, so that part of the problem was being caused by the multiple Joomdle plugins being invoked in error.

The final order processing still seems a little slow, but it's acceptable. Thanks for your and Nocolas' help on this.