We are having the same problem after upgrading to Power BI Report Server May 2019. Power BI creates a tabular model of your data in an embedded SSAS instance when the report is run and should then give the user read access to this model.
In the May update, this permissions assigning process doesn't seem to be working which is why your error message says that you can't connect to the SSAS server. We've also seen that the default mode of the model is changed to 'Import' instead of 'DirectQuery' when we query the SSAS instance from the report server.
Our workaround is to re-install the January version of PBIRS desktop which seems to work OK even with May's version on the server. Hopefully MS can issue a fix so we can use the new May features again soon.