I worked around the issue... but wanted to report this error. i found it very odd... this is an incredibly simple site, with very few plugins activated.
I believe that I realize what caused the issue... so FYI, i was upgrading Hummingbird from pre v2 to current version. seemed to be missing a check on the advanced-cache.php file?
[01-Jun-2019 17:06:03 UTC] PHP Fatal error: Uncaught Error: Call to a member function write() on null in /wp-content/plugins/wp-hummingbird/core/modules/class-module-page-cache.php:1522 Stack trace: #0 /wp-content/plugins/wp-hummingbird/core/modules/class-module-page-cache.php(274): WP_Hummingbird_Module_Page_Cache::log_msg('[17:06:03] Load...') #1 /wp-content/plugins/wp-hummingbird/core/modules/class-module-page-cache.php(908): WP_Hummingbird_Module_Page_Cache::load_config() #2 /wp-content/plugins/wp-hummingbird/core/modules/class-module-page-cache.php(1326): WP_Hummingbird_Module_Page_Cache::should_cache_request('/explore/wp-log...') #3 /wp-content/advanced-cache.php(32): WP_Hummingbird_Module_Page_Cache::serve_cache() #4 /wp-settings.php(95): include('/home/jdubsent/...') #5 /wp-config.php(107): require_once('/home/ in /wp-content/plugins/wp-hummingbird/core/modules/class-module-page-cache.php on line 1522