Last week, I noticed an email that was scheduled and sent successfully was still listed in the Emails menu in the “Sending” category, but I didn’t look into it too much since it did in fact send. Now, I wonder if this is related.
Today, I tried replacing ALL of the inbound-pro files (with the most current version, of course) and I also tried cleaning the database using WP-Optimize (https://wordpress.org/plugins/wp-optimize/), but I am still getting this error.
Recently trying to work inSparkPost EU support has caused a few problems with datetime formatting that hopefully I have fixed with the latest release. Will you update and check if everything is back to normal?