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When somebody sent me text messages by Spark, the window of Spark appears but I don’t see any text.

If you close Spark down and re-open it again and ask somebody to retyp his text message, then you can see the text messages, somebody retyped, again.

We are using Spark version 2.6.3.

What would cause this?

It is a bug, which since has been fixed in 2.7.0 version (and newer ones).