Hello and thanks for the reply.
It seems the main problem wasn't a powercli problem but it was caused by the specific template I was using. I tried another template (since this was the only thing I didn't change for debugging) and this works without a problem. Even connecting through HTTPS.
The original template is a Debian 6 VM which was made in vSphere 4.1 (guestid: Other 2.6.x Linux 64-bit) and I changed this with the upgrade to vSphere 5.0 into Debian GNU/Linux 6 (64-bit). I will just play safe and recreate this template using the "new machine" wizard.
Problem solved in a weird way.