Hi dke!
I cant tell if its an NI issue or Sonar, to be honest I havent ever dared freeze and unfreeze a Guitar Rig stack before...
I will send through the offending Project File to the Bakers so they can take a look.
...Just a word of caution to all Gtr Rig 5 users (maybe previous versions as well but I dont have Grig 4) ALWAYS save your stacks/rigs and be careful of freezing, maybe save a project as a different name type, reopen and then freeze...i.e backup before freezing/unfreezing.