The itemprop="image" property of DiscussionForumPosting schema showing wrong image url.
I can also reproduce this in Discourse meta.
For example, in this URL https://meta.discourse.org/t/can-i-increase-the-1024000-kb-limit-on-attachment/165367 itemprop image URL showing as https://meta.discourse.org//assets-meta-cdck-prod-meta.s3.dualstack.us-west-1.amazonaws.com/original/3X/c/d/cd3fa05cda365614f7ec6f93fbca4974decb16f0.png which is an 404 error page.
I found this error after seeing so many 404 crawl errors of my forum in google webmaster tools. Because every one of my forum topic has at least one image.