ADF copy data in for-each loop can't find server now and then

Hi,
I have a simple loop executing serveral copy table activities over the same connection(s).
Almost daily one of the activities fails saying:
"ErrorCode=SqlFailedToConnect,'Type=Microsoft.DataTransfer.Common.Shared.HybridDeliveryException,Message=Cannot connect to SQL Database. Please contact SQL server team for further support. Server: 'xxxxxxxxxxxxxx', Database: 'xxxxxxxxxxxxxx', User: ''. Check the linked service configuration is correct, and make sure the SQL Database firewall allows the integration runtime to access.,Source=Microsoft.DataTransfer.ClientLibrary,''Type=System.Data.SqlClient.SqlException,Message=Login failed for user '<token-identified principal>'.,Source=.Net SqlClient Data Provider,SqlErrorNumber=18456,Class=14,ErrorCode=-2146232060,State=1,Errors=[{Class=14,Number=18456,State=1,Message=Login failed for user '<token-identified principal>'.,},],'

I don't understand, what can I do?