There is a subtle issue when linking parameters in xspec. If you do something like
XSPEC12> newpar 5 = 12
then the hard limits used for parameter 12 are those of parameter 5. This is not usually a problem however if parameter 12 is part of a table model and its tabulated parameters cover a smaller range than parameter 5 it is possible for the fit to reach a point outside the available tabulated data. This will generate a warning message in v11 and an error message in v12.
The best workaround is to always make sure that table model parameters are linked to rather than from so that the hard limits in use will be those from the table model.