data_engine.log shows
de: COM Error [0x80040e31] IDispatch error #3121 - [Microsoft OLE DB Provider for SQL Server] Query timeout expired
...seems related to the problem. It makes no difference if you use the old default data_engine provider or the OLEDB provider, both generate the "timeout" message in the log.
When working on a lot of dashboards - the response time is sometimes terrible.
It's almost the impression that some new code is "locking" the sql database.