"An unknown error occurred" in assets screen
I got an emailed response from Andris from P&amp;T: 


  That indicates that at some point Craft was unable to index a file,
  leaving some corrupt data. If you look at craft_assetfiles table,
  you'll notice that at least one entry has the dateModified column set
  to null. Removing those should fix the issue for you. Probably the
  best way to remove them would be to actually delete the records from
  the craft_elements table, that have the matching IDs. The cascading
  delete should remove all related records.
  
  Please do make backups before tampering with the database, though!


Deleting the records fixed the issues.