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:
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)