Hi Stanislav ,
Can Tableau Server access those folders ( excel files ) ?
Can you create a separate dummy workbook with only SQL DB as datasource and try refreshing it ?
If data is getting refreshed properly at Tableau Server --> means you have access problem to that folder from Tableau Server (Not from your desktop though )
You need to make sure your server Run As Acct has access to the network folder where the excel is saved. Check out this link: "Database error: 0x80004005: Microsoft Office cannot open or write to the file" Error | Tableau Software