I have a very frustrating problem. I've spent over half hour on this with no success.

I can save the blog post, until I try to link to https://www1.maine.gov/dacf/mfs/projects/kennebec_woodlands/downloads/documents/estate_planning_checklist.pdf

As soon as I try to add that link to some text the post - previously complete, refuses to save and wipes 2/3rds of it.

Why should this not save? It's a valid and resolving URL.