I'm in the process of migrating our Confluence 5.10.8 installation to a new Confluence 6.13.11 instance on Ubuntu 18.04. In the process, I'm also utilizing a new Postgresql 9.6 instance. The old Confluence 5.10.8 site used a Postgresql 9.4 instance.
I exported the old site to xml, and copied the entire attachments folder to the new server in the Confluence Home directory.
However, I'm now encountering 2 severe issues in some entire Spaces and selected pages. As far as I can tell these issues are mostly happening in just pages that haven't been updated in at least 3 1/2 years:
I tried re-migrating just one of the Spaces over, individually, via an XML backup, and that seems to have resolved the issues in that one space.