Power BI Scheduled refresh error

About from a week ago, scheduled refresh has been failing due to the following error. normally it fails due to this error

"efore the data import for finished, its data source timed out. Double-check whether that data source can process import queries, and if it can, try again."

but I get this error too

"Invalid connection credentials.".

I've checked the gateway connection and confirmed that data source crendentials are all correct. 

it started wih one dataset but now multiple datasets are impacted :(. when I refresh manually on power bi desktop files, they all work fine.

any help would be much appreciated. cheers