Here we can see that its a sessions error, judging by The Imagevue FAQ (
viewtopic.php?t=4461 ):
Error: Failed to write session data
This happens every so often, session data is some small info to identify user who is logged in. This is stored on server, in temp directory. But sometimes this is either misconfigured or just directory not created or unwritable. We can't fix it, this should be forwarded to your hosting support.
sessions:
viewtopic.php?p=12261#12261