Short check list:
1. Check if the extension is compatible with the Magento version you just upgraded to.
Usually we try to keep our extensions compatible with latest version of magento but sometimes when major changes occur in magento core compatibility adjustments may take some time.
2. Try upgrading the WeltPixel Extension to latest version available on weltpixel.com.
We constantly release updates with new features and fixes. Maybe the fix is in the latest extension version and you don't have to debug further.
3. Try clearing Clear all Magento Caches.
4. If you have other extensions installed maybe another 3rd party extension is generating a conflict in latest Magento version and breaking functionality for other extensions.
Try to take a look with the browser Inspect Element ( Right Click + Inspect Element) and see if you have any errors in Console. If yes they should probably be addressed as any error there may affect extension functionality.
Magento known issues:
1. Make sure the issue is not Magento related as Magento still has known issues, you can check Magento Open Issues on Github.
1. Refer to this article for more details on Choosing the right Magento 2 hosting partner.