This issue can be caused by a build up of idle tasks which also stop the maintenance from running each night, which in itself, compounds the problem. Idle tasks are tasks which have completed but which have not cleared down the "iwtskrun.exe" process on the server.
This can be prevented by adding some registry keys on the server, which ensure the process does clear down.
Contact our support team to get the registry keys added, or to receive the details to share with your IT team.