Problems with Varnish caching on Dreampress
Then the description of the problem is wrong. It comes from other incompatibilities not from cache headers. I'm sorry but I have no idea what can be the reason of these two plugins incompatibility, we're not familiar with the Dreampress Serve and with the caching solutions you use. We have no chance to help in such situations. The 3rd party system incompatibility is not included in our support. If forum is too important for you, just change the cache solution or hosting service.
Enable WP Debug.log and check for fatal errors. Maybe some details in there help you.
WP_DEBUG_LOG is a companion to WP_DEBUG that causes all errors to also be saved to a debug.log log file inside the /wp-content/ directory. This is useful if you want to review all notices later or need to view notices generated off-screen (e.g. during an AJAX request or wp-cron run).
Note that this allows you to write to /wp-content/debug.log using PHP's built in error_log() function, which can be useful for instance when debugging AJAX events.define( 'WP_DEBUG_LOG', true );
Note: for WP_DEBUG_LOG to do anything, WP_DEBUG must be enabled (true). Remember you can turn off WP_DEBUG_DISPLAY independently.
WP_DEBUG_DISPLAY is another companion to WP_DEBUG that controls whether debug messages are shown inside the HTML of pages or not. The default is 'true' which shows errors and warnings as they are generated. Setting this to false will hide all errors. This should be used in conjunction with WP_DEBUG_LOG so that errors can be reviewed later.define( 'WP_DEBUG_DISPLAY', false );
Note: for WP_DEBUG_DISPLAY to do anything, WP_DEBUG must be enabled (true). Remember you can control WP_DEBUG_LOG independently.