Hi Team,
So I have updated my system as well to 3.9.0, I have also encountered the error that everyone is describing (Call to undefined method Piwik\Plugin\Manager::getPluginDirectory{}), After going back to Matomo root, the system was working as usual.
I have today noticed a glitch however; Month filter doesn’t display data for the current month anymore.
I have tried other filters, (Week and Day), they all display the data correctly on my Dashboard.
Any idea where this could come from ? Looking fw a 3.9.1 
Thanks a lot!
EDIT: After like 30 min, I could see the numbers are back for the current month…I suspect some background job (cron?) to run slower than usual, just a guess…