How to enable DEBUG logging on server?

Before reproducing the problem it makes sense to enable 'DEBUG' log level for TeamCity classes.
On the server side, go to the Administration | Server Administration | Diagnostics page and select debug logging level ('debug-all', 'debug-vcs', etc).

After that, DEBUG messages will go to teamcity-*.log files (read more).

For detailed information, please refer to the corresponding sections:

TeamCity Server Logs
Viewing Build Agent Logs