Hi Alexey,
Are you using AR8 reports with AR8 Server or are you using the Report Designer Add-on for designing the report that you are running?
The Report Designer Add-on uses the Data Model technology in ActiveReports 8 Server. The SQL command timeouts for running the reports are setup in the Data Model properties.
Here is more information about where this is set:
http://helpcentral.componentone.com/nethelp/AR8Help/ARS8_Administrator/#ChangingAConnectionString.html
Let me know if this helps. If you are still getting this problem, please run the Diagnostics from the admin dashboard and share the output file from there. That will help us investigate further.