pdflatex won't write in certain folders

Question

I have TexLive on Windows 10 installed. For some reason, it can only write to files in certain folders, and not others.
I am using the command: pdflatex sample2e.tex
This uses a sample document that comes with TexLive for testing. The file can be found here: http://www.wikiupload.com/UFRD0WK19701I6J

I get the following output in these folders (i.e. it works): C:\Users\josh_,C:\Users\josh_\testing, C:\testing

This is pdfTeX, Version 3.14159265-2.6-1.40.18 (TeX Live 2017/W32TeX) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(c:/texlive/2016/texmf-dist/tex/latex/base/sample2e.tex
LaTeX2e <2017-04-15>
Babel <3.13> and hyphenation patterns for 84 language(s) loaded.
(c:/texlive/2016/texmf-dist/tex/latex/base/article.cls
Document Class: article 2014/09/29 v1.4h Standard LaTeX document class
(c:/texlive/2016/texmf-dist/tex/latex/base/size10.clo)) (./sample2e.aux)
(c:/texlive/2016/texmf-dist/tex/latex/base/omscmr.fd) [1{c:/texlive/2016/texmf-
var/fonts/map/pdftex/updmap/pdftex.map}] [2] [3] (./sample2e.aux) )<c:/texlive/
2016/texmf-dist/fonts/type1/public/amsfonts/cm/cmbx12.pfb><c:/texlive/2016/texm
f-dist/fonts/type1/public/amsfonts/cm/cmex10.pfb><c:/texlive/2016/texmf-dist/fo
nts/type1/public/amsfonts/cm/cmmi10.pfb><c:/texlive/2016/texmf-dist/fonts/type1
/public/amsfonts/cm/cmmi7.pfb><c:/texlive/2016/texmf-dist/fonts/type1/public/am
sfonts/cm/cmr10.pfb><c:/texlive/2016/texmf-dist/fonts/type1/public/amsfonts/cm/
cmr12.pfb><c:/texlive/2016/texmf-dist/fonts/type1/public/amsfonts/cm/cmr17.pfb>
<c:/texlive/2016/texmf-dist/fonts/type1/public/amsfonts/cm/cmr6.pfb><c:/texlive
/2016/texmf-dist/fonts/type1/public/amsfonts/cm/cmr7.pfb><c:/texlive/2016/texmf
-dist/fonts/type1/public/amsfonts/cm/cmr8.pfb><c:/texlive/2016/texmf-dist/fonts
/type1/public/amsfonts/cm/cmsy10.pfb><c:/texlive/2016/texmf-dist/fonts/type1/pu
blic/amsfonts/cm/cmsy7.pfb><c:/texlive/2016/texmf-dist/fonts/type1/public/amsfo
nts/cm/cmti10.pfb>
Output written on sample2e.pdf (3 pages, 136743 bytes).
Transcript written on sample2e.log.

However, I get the following output in these folders (i.e. it doesn't work): C:\Users\josh_\Documents\tester, C:\Users\josh_\Documents, C:\Users\josh_\Dropbox, C:\Users\josh_\Dropbox\tester, C:\Users\josh_\Desktop, C:\Users\josh_\Desktop\testing

This is pdfTeX, Version 3.14159265-2.6-1.40.18 (TeX Live 2017/W32TeX) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
! I can't write on file `sample2e.log'.
(Press Enter to retry, or Control-Z to exit; default file extension is `.log')
Please type another transcript file name:

Can you tell me what is going on and how to fix it?

I particularly want to work in my Dropbox folders.

Thanks

Answer 1

My antivirus/firewall program Bitdefender Total Security 2018 has the above "failed" folders protected. Upon opening the program it gave notifications that it had blocked pdflatex from writing to certain protected folders.

I then told Bitdefender to allow pdflatex to write to protected folders by doing the following:
1. Open Bitdefender.
2. Go to the main screen (icon is a "B" inside a shield).
3. Click on View Features.
4. Click on Application Access in the Safe Files box at the bottom right of the interface.
5. The blocked program(s) will be listed as "blocked". Click the toggle switch to set them to "allowed".

After this, pdflatex could write to the "failed" folders listed above.

Thanks to everyone for your help! A big thank you to Ulrike Fischer for the key insight of looking for a program that is protecting the folders!