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I see in my log the nexe warning:

[2017-05-05 12:23:53] main.WARNING: Attempt to load value of nonexistent EAV attribute '339'

                        for entity type 'MagentoCatalogApiDataProductInterface'.  

I can't find why, but i find this and i don't know if this is true.

Do you know why this problem occurs?

Make a database backup.

Go to the database and check in the eav_attribute and the other eav_* tables if you find a reference to that id.
You could try to delete that reference if that attribute was deleted.

This information is supplied without liability. I never did that. It's just on top of my head.

I have this issue and know how it was caused in my setup. We moved products over from another Magento2 install. The ID numbers for the EAV attributes will have likely been different. So while the attributes are all still there and assigned to the same products. The entity_id number will be different. Shy of trying to marry up the id numbers again back to their original values, I'm not sure how to fix this..... bit messy! In your situation, if you have a previous version of the eav_attribute table, try restoring the previous version and see if the error's in your log remain.

Note: Backup your database first.

Find the string Attempt to load value of nonexistent EAV attribute in our project. On my current Magento version 2.2.2, foud it in

vendor/magento/module-eav/Model/ResourceModel/ReadHandler.php

foreach ($connection->fetchAll($unionSelect) as $attributeValue) {

                if (isset($attributesMap[$attributeValue['attribute_id']])) {

                    $entityData[$attributesMap[$attributeValue['attribute_id']]] = $attributeValue['value'];

                } else {

                    $this->logger->warning(

                        "Attempt to load value of nonexistent EAV attribute '{$attributeValue['attribute_id']}' 

                        for entity type '$entityType'."

                    );

                }

            }

We need to dig into this part with xDebug:

Check entity_id in eav_attribute and check backend_type. For example: int, datetime, decimal, varchar...

And then find the table of this type: catalog_product_*

I migrated from M1 to M2 and had similar problem.

I fixed it by adding the missing attributes to the attribute set created by migration tool.

Find the attribute code by running following query:

SELECT * FROM `eav_attribute` WHERE `attribute_id` = 339

In the result you should see the attribute code. Now log in to the admin panel and navigate to Admin > Stores > Attributes > Attribute Set

Find the migrated attribute set (Usually looks like this Migration_[AttributeSetName]) and add the attribute code to the attribute set.

This solved my problem that appeared after migration.