I have this problem on my both Macs. 1st is my personal Macbook Pro 15" 2014, used a lot. 2nd is my working Macbook Pro 15" 2015, right from the box, unused. No torrent software, no extensions for Safari, no antivirus. And on both after updating to Mojave I had this problem – I can't open saved page when I'm offline. And yes – safe articles for offline reading is turned on. This is the error log, I got like 3.2K messages in 10 seconds.