Last weekend we had this annoying problem were SharePoint Designer 2013 refused to open SharePoint Online 2013 Site Collections:

Mind the: “Was this information helpful”, no not at all…

After some searching on the internet I found, change web.config settings or do a iisreset, uhm this is SharePoint Online!

Frustrating, till the suggestion in a comment:

[fancy_box]DELETE THE SITE COLLECTION LIST TEMPLATES! [/fancy_box]

After > Site Settings > List Templates > “Delete every single List Template”

SharePoint Designer 2013 opens the Site Collection again like it should…

Who would’ve thought ლ(╹◡╹ლ)