@keabk No unfortunately I haven't found a fix for my problem. And allthough you see the same error I'm not sure they are related. Mine was an error in Plone 4 with the Zserver implementation, your error is occuring in Plone 5.1 or Plone 5.2 with the new server implementation
When does your error occur, when you stop the zeoserver instance? It seems one part of the server is already closing database files (and the cache from the error above) and nother is still trying to load objects from the database where the _storage object no longer has a connection to the database. But this is high level guessing.
(This was one of the errors in plone.app.async why I recently advised in another post on this forum (Calling (slow) rest from Plone) not to use plone.app.async anymore if you need async tools but something like collective.taskqueue which is more active in use, simpler and somewhat maintained. )