Ok, so the messages are unreadable. I haven’t used used OTR for a long time. Actually it might be even removed from the next version of Spark, as it was not developed for a long time and there is no updated library in maven format to be used with current Spark source.
On your issue. I think it might happen when one of the users close their chat window. OTR session is closed then, even when the other user still has it open. So when that other user sends messages, they are still encrypted, but the first user has his OTR session closed, so Spark is not decrypting them anymore. A workaround is only to instruct users not to close chat window when using OTR and close chat windows on both sides and reestablish OTR session, if one of them closes the window.