There's a warning in the Confluence log:
WARN [http-nio-8090-exec-3] [webresource] writeTo error in 'Content.writeTo' for de.communardo.confluence.plugins.subspace:
There is no other faulty behavior by the app.
You have encountered an existing bug in our app. The bug does not affect the performance of the app or Confluence and will be fixed in one of the next releases.
It is being tracked in our ticket system:
CSSP-470Getting issue details...