Similar issue. Has anyone found a solution yet? Our site was fully indexed till feb and offline for 2 months. Upon restoring with backup, its back on google fully. But Bing refuses to index citing some problem "The inspected URL is known to Bing but has some issues which are preventing indexation. We recommend you to follow Bing Webmaster Guidelines to increase your chances of indexation."
URL is https://www.immiboards.com
We have raised a ticket with Bing webmaster tool today.
