Introduced by commit 6855d9d461e76978e59f4b8f9ff17d5a97c96f71 / OF-1835. Whenever a detached session is being resumed, and debug logging is enabled, this stack trace is logged. It's caused by attempting to log the session instance (which only happens when the debug log level is enabled):
The problem is logged on level 'error', but is otherwise ignored. The impact of this should be minimal.