Bathroom exhaust fan without duct
Is this a big problem? I assume the fan has been like this for 15+ > years. If it is, I will look into getting a duct there, which is complicated by the inaccessible attic space.


Moisture in the attic is a problem and the fan needs to be ducted to the exterior. I find it hard to believe that your attic has no access. Code requires one for many reasons including maintenance and for fire fighting. 


  Should we stop using the fan until this gets fixed?


After 15 years, a little longer won't be a big difference in the overall picture and not using it can cause problems in your bathroom due to excessive moisture there. 


  Should we be concerned about any mold etc in the attic as a consequence of this?


Yes. When you get the work done have them check for mold and decay of the roof framing members and roof sheathing