This is the standard error caused by FLUENT trying to access data that has not been allocated.
There are several causes discussed below.
For example, in a UDF, you may probe the cell temperature using the macro C_T(c,t). However, if you have not turned the energy equation ┐on┐, then there is NO temperature stored, and you will get an ACCESS_VIOLATION.
To check a Thread for whether a variable is stored on it, you can use the THREAD_STORAGE(t, SV_XXX) macro, where SV_XXX is the storage variable for the equation of interest. For the energy equation, you would use SV_T. This macro returns a pointer to the data. The value is NULL if the data is not available. See ┐storage.h┐ for a complete list of storage variables.
Another cause for the macro is that your case has a UDF hooked, but you do not have the UDF available. The solution is to make sure you have the UDF directory in the correct location relative to the case file.