This case happens when Tables are looking for the original variables stored in the .ddo file, but they're not available in the .ddf2 file. How could that be possible that certain variable are no more available in the datafile (.ddf2 file)?
The most common cases are:
+ .ddf2 file is different from the one used to produce the .ddo because some changes have been made and for example 1 or more calculated variables are not more available.
+ .ddf2 file is different from the one used to produce the .ddo because some changes have been made and for example some variables have now a different variable name.
+ .ddf2 file is different from the one used to produce the .ddo because some changes have been made and for example a multi-dichotomous or multi-response variable is not computable because the base variables don't exist anymore or changed names.