Data refresh stops w/o any message

My Dataset in Power BI Desktop is currently refreshed manually with two excel-file as source. The refreshing worked until yesterday. Since, the system starts the process to refresh data but starts an endless run w/o giving any error message or failure report.

Before the issue appeared for the first time, I converted the format of one column in the table from "text" into "date".

Can this the reason that the refreshing does not work anymore and what could be done to fix it?

Thank you!