Cannot estimate this build. Build agent "***" is restarting

Hello,

I cannot force Team City to start the build.
When I move mouse over "???" of the Build Queue table popup window appears with text "Cannot estimate this build. Build agent "***" is restarting".

What is the reason? I restarted agent two times. It does not help. Is there some flag which should be reset somewhere?

Best regards
Dima