TeamCity Server does not assign jobs to Agents

Hi,
   I see that sometimes, the server fails to assign jobs to available agents. When I check the server logs, I see messages like thread-dum took too long to complete and a related thread dump followed by nothing. Each time, I have had to restart the server to get things going again. The thread dump does not seem to have any significant crashes as well. I looked into all other logs in the agents as well as the server but all of them are inactive. I faced the same issue on 9.1.4 as well as 9.1.5. Can anyone provide some leads into this?

Thanks