VAST error 900, aka VAST 2 error, is an error that can be caused when any VAST 2 error occurs, even if VAST 3 is requested or your network default is set to VAST 3. To fix the error, the publishers either begin to request VAST 3 or enable VAST 3 for the network and the buyer should work with the publisher to troubleshoot the exact issue, or respond with VAST 3