Hi,
I take a look at your configuration.
Because you're using groups, I set some "vendor access" in the HikaMarket backend (a copy from the default accesses you set).
In the front-end, I created to create a product and when I pressed "apply" I got a message telling me that the field "Author" was required.
You have a custom field which is required but that field is only available for some specific categories and is not available in the front-end edition.
So I noted that issue in my TODO list for the next release ; I'll reproduce it in my local website and find a patch (but maybe it will affect some HikaShop files too).
For the test, I switch the "author" custom field not required and I save the product.
There is currently an issue in HikaMarket 1.6.3 which is fixed for HikaMarket 1.6.4
When you save a product with the "save" button and you got an error, you are not redirected to the edition of the product and you just got that blank screen.
The patch is on the "component/com_hikamarket/controllers/product.php" file where you have to replace
public function save() {
if(!$this->store())
return false;
By
public function save() {
if(!$this->store())
return $this->edit();
But more than the patch, you have to remove the "required" on the "author" custom field for the moment.
I hope that I'll be able to come back fast with a patch ; the issue is only when you create a product with required custom field applied on specific categories. Because the product does not already exist, it does not already have a category so HikaShop is applying him all custom fields...
I will see if I can find an appropriate patch in HikaMarket but it is possible that HikaShop might require a patch for the custom field management. For the moment it's too soon to provide a complete analysis but I'll talk with Nicolas for that.
Regards,