Hi @MariusT ,
Maybe you can provide some details or screenshots about this error (without sensitive information)？
Please try in the model designer side, you can delete tables in your model workspace database that you no longer need. Deleting a table does not affect the original source data, only the data that you imported and were working with.
Or you can delete the data source from the old PowerBI Desktop file by going to Query Editor > Open Data Source Settings > Select the source you want to delete > Select "Change Source". Delete the previous data source Reconnect to Power BI Desktop and republish to Power BI Service.
Please check this to see if it helps.
Hope it helps,
Community Support Team _ Caitlyn
If this post helps then please consider Accept it as the solution to help the other members find it more quickly.