Magento – Undefined index encrypter.php after admin login in magento 2

I had the same problem.

After searching the source of Magento 2.2.4 I found the function in file (vendor\magento\module-catalog-inventory\Model\Configuration.php):

public function getDefaultScopeId()
{
    // TODO: should be fixed in MAGETWO-46043
    // "0" is id of admin website, which is used in backed during save entity
    return 0;
}

In my case I modify Store parameters at "Stores > All Stores" and my actual store_id/scope_id was 1 and not 0. After changing "return 0" to "return 1", my problem was solved.

As this is anyhow TODO for Magento I expect, that in next upgrade this will be solved and all my modification will be overwritten and everything should work at that time...

use this code instead

* @return IndexInterface
 */
public function getFirstMatchedIndex()
{
    foreach ($this->getIndices() as $index) {
        if (($index->getData('store_id') == false
            || $index->getData('store_id') == $this->getCurrentStore()->getId())
        ) {
            return $index;
        }
    }

    if(count($this->getIndices()) > 0){
        return $this->getIndices()[0];
    }
    return array();
}

This should work.