Thanks svoutdoors media& carolakashorty. Flattening or Rasterizing the layer fixed the issue for me!
I found out the "Could not save as 'filename' because of a program error." issue was caused by one layer (found by trial and error)
Once I found the layer preventing me from saving to PDF, I rasterized the layer, and I was able to save as PDF again.
Not sure what caused the layer problems, but importing the layer from another PSD didn't help. Rasterizing or Flattening the layer was the key for me.
Cheers!