Is your POSTBUILD installed formally along with the rest of the NX?
The syslog should be in UGII_TMP_DIR, normally C:\temp. Or you can find that out when you display the syslog from a NX session. The first 2 lines should say something like:
*** system log created by ling on Mon Oct 15 16:52:15 2018 Pacific Daylight Time
which indicates the location of the file. "XXX" will be your login ID.
You can have the folder open before you start PB; then, you will see a new syslog getting created. I'm not sure how or why your syslog gets deleted automatically.