Couldn't load the schema for the database model

Hi, 

We imported data into our report from a sql database and published it to Power BI Service. We embedded this report to a website. We have not setup a connection between Power BI Service and our database because this is not possible and we use imported data so we thought it wouldn't be a problem. The report is working most of the time, but sometimes all the tiles in the report give the following error: 'Couldn't load the schema for the database model'. It looks like this error is totally random and when looking at the report a few hours later it is working fine again. We are using Power BI Premium EM1.

Does anyone know why this happens?