This looks like the same issue that is mentioned in this bug report. Try reading the messages out of the rosbag using: msgs = readMessages(bag2, "DataFormat", "struct");
This should allow you to access the data still, though the messages will be structures rather than classes. It has the added benefit of being faster to extract the data from the rosbag. If you have any specialized messages you want to use the convenience methods to extract data from (e.g. readImage, readXYZ, etc.), you can copy the data from the struct message into an empty message object and call the method using that. We've seen this more commonly with compressed rosbags. If it's an option, and the "DataFormat","struct" isn't enough for you, consider trying to record the rosbag in an uncompressed format.
-Cam
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