The cash register communicates with your shop via the default Shopware REST API. Security settings on your server or with your hosting provider can block this connection. Therefore, check the following points.

If you protect your shop with directory protection, for example via .htaccess, exclude the path /api from it.

Bot protection can specifically block the upload of the documents. The cash register then displays the message Rechnungserstellung fehlgeschlagen. Die REST-API des Shops ist nicht verfügbar. In one case, Cloudflare responded to the endpoint _action/pickware-pos/upload-document with the state 403 and the page "Just a moment…". Adjust the rule of your bot protection so that the Pickware endpoints are reachable.

If normal sales on the cash register fail after the database has been moved or the database user has been modified, database triggers may still point to the old user. Pickware uses the database user that is stored in the Shopware configuration or in the .env file. In this case, contact support so that the triggers can be recreated. Alternatively, create a user with the old name and all permissions in the new database again.

If your shop is not reachable or only reachable very slowly, the cash register cannot log in. As soon as the shop is reachable again, the cash register works again as well.