I have no problem running pdflatex outside of LXDE - actually I hardly use LXDE at all.

Perhaps your issue is that your working directory is "/" when you run pdflatex and then you might be in danger of running out of disk space if you have filled your Debian image to the brim! But if you really run your commands by the one line you wrote, it might have just tried to process everything in "/" and failed because user cannot write there.

Try debbie-sue instead of debbie, that will give you /home/user as default working directory. If you want to use some designated directory for your LATeX processing, optimally in MyDocs, you should better first enter a debbie terminal, change to that directory, and then do your processing. Or define a shortcut as described here to be able to run with one line from XTerminal.