Service Account - Credentials to refresh data, error by Refresh

Hi, 

I would like to understand something since me and my colleagues are using a common Service account to schedule the refresh of our data sources in Report Server.

The SVC is authorized correctly on every SQL Server, Folder or source we are using.

The data refresh seems to be working just partially, sometimes it happens succesfully, several times it throws the error.

The below is an example of it, I didnt change anything between the 2 schedules, same da different results.

It seems to be some kind of Queue error, probably too many Reports are due to refresh at the same time, but I can't find any info about it on the web. Is there somebody who knows about this issue and the related solution?

Many thanks