from the server log it looks like others had that problem too.
There is a simple workaround:
Get yourself killed by the enemies, after that a new round will start in the build phase.

it seems that the engine variable "num_clients" does no longer get set to 0, although there is no one on the server. I'll post a bug report soon.

Edit:Version V0.55 is now released, please look at the first post for details.