I am trying to mine on the testnet. Unfortunately, my Ethereum Wallet (latest version using Geth 1.5.3) kind of gives up doing anything when I click on start Mining on testnet. The time counts up is displayed in red, but nothing gets synchronized anymore.
In the "normal" non-mining mode, i've got up to 25 peers and actual blocks get synched from time to time (every ~20-40sec).
What could go wrong here? And how can I find out?