Recently, we upgraded a dev server from SDL Tridion 2013 to SDL Web 8.5. The dev server also had an old version of Alchemy installed, so we upgraded this one as well (to the latest 1.0 version).
In the event viewer, we noticed a stack trace that hinted that alchemy was trying to connect to the Session Aware Core Service using an empty username.
After some more searching, we found that the Alchemy virtual directory had both anonymous access and Windows Authentication access enabled. After disabling this anonymous access, the problem was solved - alchemy was now using the username of the logged in user to connect to the Core Service and the errors were now gone.