Bearing in mind I know next to nothing about PXL expressions...
I just added one and Blend&Comp'd it with the existing stream (just before the Output node) and it worked fine (AFAICT anyway). Tried several of the presets with no issues.
Have you applied the latest SP? (SP2)
Try un-muting the Develop section and see if the text looks sensible - could be a corruption of the expression files. They're in \resources\toolPresets\PXL\* if you want to check them outside of Composite.
Max 2016 (SP1/EXT1)
Win7Pro x64 (SP1). i5-3570K @ 4.4GHz, 8Gb Ram, DX11.
nVidia GTX760 (2GB) (Driver 430.86).