Your comments

Looking at the second image you've posted, that warning is a non-fatal one... It appears to create the thumbnail just fine, after a hard refresh does the thumbnail appear?

Not seeing any errors in that log, may be more info in the PHP log

Heres the latest working ghostscript aswell if you want that too... fairly certain the path needs to be added to the environment variables, i.e. C:\Program Files\gs\gs9.52\bin

Oh and the fonts... should be placed in the gs folder

Try Graphics Magick from here, versions newer than 1.3.35 dont work

Did a completely fresh install and wiped database

Same issue

Issue resolved
PHP Docs say here that on IIS cgi.rfc2616_headers must be set to 0 for basic auth to work

Ok so heres what I did to get STL thumbnails

Download stltopng and place it in your cron colder

Download Papa’s Best STL Thumbnails and install with powershell (run as admin)

msiexec /i "Papas Best STL Thumbnails.msi" MSIINSTALLPERUSER=""

Now create a batch file in your cron folder containing 

"%~dp0stltopng" /res 400 /png "%4" "%3"

Now you can point filerun to this instead of stl-thumb, working flawlessly here!

It would appear as an app Tika requires access to C:\Users\<username>\appdata\local\temp

So whatever account Filerun runs under will require access to this folder

Procmon is a good way to figure out if an app needs access to something

Vlad, thats a good start (and an option that should be documented!) but cURL times out now

Would be nice if it were put in a foreach loop with each page rather than throwing a large PDF at it causing Java to eat 4GB of ram

Using tika server here, seems to be working fine

Steps I took to enable:
Get latest Java + Tika server jar 
set environment variable JAVA_HOME to C:\Program Files\Java\jre1.8.0_341

Create batch file containing 
java -jar "C:\path\to\tika\server.jar" --port 9998

run batch file as scheduled task

Only issue I've had is with large pdfs but thats more a limitation of Tika + Filerun should be going by sections for large pdfs