Joining chat room in Spark returns "no response from the server"

I have two Group Chat rooms that were working fine up until (I believe) today. I don't use them normally, but I haven't had complaints from regular users until today. I can see the rooms listed in Spark, but connecting to them almost always returns "No response from the server". After trying to join multiple times, I can sometimes get in eventually, but it always gives me this error.

I normally use Pidgin, and I don't see any errors or experience any issues joining these rooms manually. The problem seems to be solely a Spark one.

I don't see any corresponding error in OpenFire's error/info/warning logs, and enabling the Debug log only produces LDAP lookup info, nothing related to group chat. Spark's error.log records this:

Feb 10, 2010 10:32:16 AM org.jivesoftware.spark.util.log.Log error
SEVERE:
No response from the server.:
    at org.jivesoftware.smackx.PrivateDataManager.getPrivateData(PrivateDataManager.ja va:209)
    at org.jivesoftware.sparkimpl.plugin.scratchpad.Tasks.getTaskList(Tasks.java:201)
    at org.jivesoftware.sparkimpl.plugin.scratchpad.TaskNotification.notifyUser(TaskNo tification.java:56)
    at org.jivesoftware.sparkimpl.plugin.scratchpad.TaskNotification.access$000(TaskNo tification.java:35)
    at org.jivesoftware.sparkimpl.plugin.scratchpad.TaskNotification$1.run(TaskNotific ation.java:42)
    at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
    at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
    at java.util.concurrent.FutureTask.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)