Oracle has acknowledged a bug in HFM 18.104.22.168.205 PSU and is working on a fix.
The observed behavior is you have an HFM data grid open in SmartView and you get the session expired / disconnected message well in advance of the defined session timeout setting.
I've noticed this seems to happen more frequently if your user has a bunch of VB SmartView retrieval functions on their retrieval template instead of defining a traditional ad-hoc grid. This is pure speculation on my part as I don't have too many systems I'm supporting that are on this particular patch. It seems that each VB retrieval cell maintains its own connection, and apparently once one cell gets disconnected, all of them do and your back-end XFM ODL logs are flooded with messages.
Oracle Development has indicated the bug will be fixed in the upcoming patch HFM 22.214.171.124.206 PSU.
For now, the only known workaround available for the issue is to downgrade to HFM 126.96.36.199.204 PSU.