Network or stand alone?  I saw someone in the ProSeries FB group that had this issue yesterday (it kept dropping his default printer as well) he ended up with a network issue...I didnt see if or how he got it fixed.

Im on Stand Alone and I cant print from the program after an update, I have to reboot the computer (I used to have to reboot the printer as well) before I can print from they program again.

Do you get any error message?