Petr,
Looking at the error, it could be a couple of things. I'll list three of them with possible solutions below.
You have already registered a node and client named "module8"
Each node and client must have a unique name on the Chef Server. The node you previously created may not be available anymore, maybe it was a VM that's expired or something. To fix this you'll either need to use a different node name "-N modulesomethingotherthan8" in the bootstrap command or delete the existing node and client from the Chef server. You can do this with the following knife commands:
knife client delete module8
knife node delete module8
You have an outdated validation.pem file locally
If your local validation.pem file is wrong, you'll not be able to use it to bootstrap new machines. To fix this you can download a new validation key from the Chef server web user interface.
Click on the Administration tab
Select your Organization
Click "Reset Validation Key"
This will generate a new "ORGANIZATION_NAME-validator.pem" file that you should place into your .chef directory and then try re-bootstrapping.
The client.pem is wrong on the node
If this is the case, login to the node and run this command "rm /etc/chef/client.rb" and then try bootstrapping again. You may also need to run the knife commands mentioned above for deleting the node and client.
Please give these a try and let us know if that helps.
Thanks!
Nathen