You can enable Debugger in Spark (in Advanced menu on login screen). Run Spark with it, then watch the incoming Raw data. if you see messages there, but not in the chat window, then this is related to other similar issues that were fixed in previous Spark versions, when offline messages or private messages in group chat were not being shown (but received according to debugger). I see another similar report Spark 2.9.4 : Refresh bug in a Room screen
I am guessing Openfire 4.6.1 changed something and now Spark is failing to parse messages.
Of course, you can still try clean reinstall and using a fresh Spark profile (rename C:\Users\User\AppData\Roaming\Spark to Spark_old and start with fresh profile).