Envira – Unable to ‘Add New’ Gallery After Update

I appreciate any assist that may be offered; please note I am a novice at web design, hosting a private blog/gallery for family and friends in an effort to maintain control of my content. I ran a search for this issue prior to posting and hope my post isn’t redundant.

I recently updated to WordPress 5.1 and updated all plugins (although none were Envira; I am using most current 1.6.4). I purged all caches (W3 Total Cache). I continue to have access to all aspects of my dashboard accept ‘Add New’ gallery with Envira. When I try to add a new gallery either via the left pane in dashboard (Envira Gallery > Add New) or click the ‘Add New’ button on the Envira Gallery page, I am redirected to a white page: “Briefly unavailable for scheduled maintenance. Check back in a minute.” which is displayed when on page …/wp-admin/post-new.php?post_type=envira.

I have access to all the galleries that I have uploaded to date. The rest of my site seems to operating without issue. I looked for a .maintenance-file in folders and have been unable to locate one. I have uninstalled (via deletion of the plugin folder via ftp) and reinstalled Envira without a change in behavior. I can add posts and pages, and upload media elsewhere to the site. This seems to be the only spot where the site isn’t behaving as it should.

Any assistance/advice would to remedy this would be greatly appreciated.

-Keith