I am posting my response wherever I find a topic regarding this issue, as I want to try and get it resolved and not just buried away.
I’ve had the same problem (Nov 2016) and I’m reading in other posts that the problem began after WordPress updated to 4.5 AND, the problem exists with users who are hosting with companies who provide inadequate dedicated “WordPress” hosting.
I have two separate URL’s, both using the same Total WordPress theme and Visual Composer, both hosted on shared SSD servers, yet both are having the same trouble.
Increasing memory limits in the php.ini file and deactivating plugins are not resolving the problem – this is a server issue and the problem only occurs when trying to upload multiple images at the same time into Grid using the Visual Composer plugin (Media Grid, Portfolio, etc).
I have contacted my hosting company and they have escalated my issue to upper tech support, and I will post my results when I have them.
Frank