Worked with QlikTech this morning on this issue. We troubleshooted a good while, first trying to rename the QlikViewServer and DistributionService folders (in \ProgramData\QlikTech), then restarting services to verify. When that resolved it, we narrowed to QlikViewServer, then narrowed our focus to Settings.ini. After a lot of experimenting with various configurations, what it came down to this (and this makes absolutely no sense).... the QlikView Root folder specificiation.
On this server, the name of the QlikView root documents folder is D:\QlikViewRoot
There are multiple folder mounts specified, but all are named in a similar way, but with a .XXX suffix, so things like D:\QlikViewRoot.Admin, D:\QlikViewRoot.Demo, etc.
And for reasons not yet well understood (QlikTech is doing more testing), this sort of combination seems to trip the Session Collaboration. By specifing any other folder for the root (D:\test, D:\QlikViewDocuments, etc.), the problem is resolved.
QlikTech opened a new case to do more testing to see if they can reproduce on their end, so we'll see. If so, this will result in a bug report.
UPDATE: QlikTech responded and confirmed they were able to reproduce, so this is being pushed up the chain as a potential bug (issue has been logged with the QlikTech Issue Analysis Team).
So just something to be aware of in the unlikely event you choose to name your mount folders and your root folder in a similar way and see this problem.