I tried several times, even with different methods (manual Octoprint installation and prebuild image even a nightly image) to install any plugins on my Raspberry Pi 3 B+. I allways get an unknown error 
Here is my log file, hope anyone could help me with this:
Traceback (most recent call last):
File "/home/pi/OctoPrint/venv/lib/python2.7/site-packages/octoprint/plugins/pluginmanager/__init__.py", line 348, in command_install
returncode, stdout, stderr = self._call_pip(pip_args)
File "/home/pi/OctoPrint/venv/lib/python2.7/site-packages/octoprint/plugins/pluginmanager/__init__.py", line 652, in _call_pip
return self._pip_caller.execute(*args)
File "/home/pi/OctoPrint/venv/local/lib/python2.7/site-packages/octoprint/util/pip.py", line 172, in execute
return self.call(command, **kwargs)
File "/home/pi/OctoPrint/venv/local/lib/python2.7/site-packages/octoprint/util/commandline.py", line 83, in call
p = sarge.run(command, **kwargs)
File "/home/pi/OctoPrint/venv/local/lib/python2.7/site-packages/sarge/__init__.py", line 1462, in run
p.run(input=input, async_=async_)
File "/home/pi/OctoPrint/venv/local/lib/python2.7/site-packages/sarge/__init__.py", line 1071, in run
self.run_node(node, input=input, async_=False)
File "/home/pi/OctoPrint/venv/local/lib/python2.7/site-packages/sarge/__init__.py", line 1187, in run_node
result = getattr(self, method)(node, input, async_)
File "/home/pi/OctoPrint/venv/local/lib/python2.7/site-packages/sarge/__init__.py", line 1333, in run_command_node
node.cmd.run(input=input, async_=async_)
File "/home/pi/OctoPrint/venv/local/lib/python2.7/site-packages/sarge/__init__.py", line 655, in run
self.process = p = Popen(self.args, **self.kwargs)
TypeError: __init__() got an unexpected keyword argument 'async'
2018-07-08 07:33:56,830 - tornado.access - ERROR - 500 POST /api/plugin/pluginmanager (127.0.0.1) 1124.93ms