This error occurs when FLUENT gets information from the Cortex (GUI). FLUENT is expecting to get the data in a specific format, but gets another. This generally occurs when FLUENT attempts to read a case file from a LATER (or very OLD) version of FLUENT. Typically, data structures are backward compatible, but only for the most recent version.
It is a good idea to update your cases every time FLUENT is released to avoid the problem associated with this type of software divergence.
If you have a very old case, such as a FLUENT 6.0 case, you may be able sequentially update the cases, but you will need all versions of Fluent (6.1, 6.2, 6.3) on your operating system.