Magento cant change product quantity in backend - php

Question

I have a strange problem in magento. Everything I change in the "Inventory" tab of a product, gets resettet after I click on "save".
Has someone had a similar problem in the past?
Is there a way to simulate the saving progress step by step, so I can see where the error appears?
Until now I have tried to:
Disable all addons
Reindex everything new
Delete the complete cache in backend/via SQL/via ssh
Delete all products and categorys
Sorry for the lack of information. If you need any information please feel free to ask.
Thank you for your help!

Answer 1

Please try below solution may help you
I tracked this down into app/code/core/Mage/CatalogInventory/Model/Observer.php.
The event catalog_product_save_after calls the saveInventoryData method which in turn calls the _prepareItemForSave method.
Around line 225:
$originalQty = $product->getData('stock_data/original_inventory_qty');
if (strlen($originalQty)>0) {
$item->setQtyCorrection($item->getQty()-$originalQty);
}
If you comment out this line:
$item->setQtyCorrection($item->getQty()-$originalQty);
It seems to correct the problem & stock levels now stay correct when saving a product whilst simultaneously creating / canceling an order on both the front & back-end of the website.

Answer 2

I figured out, where the problem was:
I have over 4000+ different product attributes. Everytime I save a product, the information of all 4000 attributes are getting transmitted over post-method. This seems to be too much, and the URL are getting cutted after a fixed value of characters. So the quantity information does not getting transmitted.
Every product has only 1-30 attributes filled with information, the rest is blank. But which attributes are filled depends on the specific product. I have choosen this method of creating an attribute for each information, because I would like to set individuality if information is searchable or filterable.
The solution was to increase max_input_vars in php.ini.
Thank you!