Clicking 'validate' in P4SL, using MATLAB R2016a/b the following error is reported:
Could not check out p4://P4CLIENT=xxxx@ipaddress:port to C:\MATLAB\Projects\ with error "Unable to get connection"
This works with MATLAB R2015, so possibly a compatibility issue. Please update the P4SL plug-in to allow it to work with the later R2016 MATLAB release.
It should support the latest version R2018b release as well.
I'm curious as to what you did to make it work in 2015 - I've tried P4SL in R2015b with a 19.1 P4D and a 15.1 P4D, neither of which work.