Works for me. Which version of Zotero and the connector?

How are you authenticating for access to the PDF (i.e., are you on a university campus, do you go through some kind of proxy, do you log in to the website)?

Also, when you say "unpredictable", does that mean that you can sometimes download the file and sometimes not? Do PDF files download for journal articles ever?

I checked the error log Report ID: 1436573065

Seems that there is some kind of javascript error coming up.

Can you provide a Debug ID from Zotero (rather than the Connector)?

And a report ID 1701697354

There is only one user on this machine and that user account certainly has admin permissions. Windows (or Zotero) also inserted some unknown user ID's. I have allowed full read-write permissions across the board for all listed users and still no go. I do not recall installing Zotero as an administrator.

Certainly see the error. But this failure only occurs when using the Zotero Connector and only when trying to download PDF files. I can drag and drop PDF files into Zotero with no problem. Also I can save PubMed pages no problem. This computer is a regular personal computer using Win10 Home.

I had Microsoft work with me to debug potential security and permission issues. From their perspective, the folder permissions are configured properly. File operations in Zotero seem to be working. The program can write to the directory (drop a PDF file into Zotero and it will correctly place the file into a new folder in the storage location) Zutilo can rename that attachment and place it into another directory. So, it seemed to Microsoft, the permission on the folder are working as intended. But none of this can occur through the browser. The question is why the browser connector is unable to access the storage location and instead reports a permission failure. The connector issue occurs in both Firefox and Chrome so it is not dependent on the browser. MS was at a loss to explain the behavior and nothing in the event logging provided any clues.

There was one question that MS had regards the behavior of the software. What persistent links are created to the destination storage folder? We attempted to uninstall and delete all remnants of Zotero. To save the current image we tried to rename the directory (called it zotero_old) but that failed. Seems that there is another process with a link to the folder. Even with the browsers and Zotero turned off, the link persisted. The only way to break the link was to restart the computer or uninstall Zotero. That was somewhat peculiar to them.

So far I have:
Reinstalled Zotero desktop and connector - twice.
Refreshed Firefox and reinsalled all add-ons
Removed security software
Performed an in place repair of MS Windows 10 - no change for any of these items.

Thanks. Checked it out but not sure if it is the same issue. I have pretty much exhausted all Windows permission fixes. I even activated the elevated administrator user and installed zotero and zotero connector for that user. Using the elevated administrator user should eliminate all permission issues. But that was a no go. Microsoft is essentially saying that this seems to be a user software issue and unless I want to go through the trouble of wiping my computer and then reinstalling all software then I have to live with the problem. Oy.

My local directory is not on a cloud device of any kind.

I have uninstalled and reinstalled the Zotero software a number of times. I have allowed it to use the default location and in those instances the sync process with the Zotero servers has worked flawlessly. So it seems the Zotero software is able to write to the local storage directory. But the Zotero browser connector has not worked under any circumstance. Still the same error: access denied.

I have tried to run the software with all plugins disabled. No change. I have also gone further to diagnose what is going on. I have run a process monitor during the Zotero connector initiated download. You are spot on correct that the connector is opening the storage folder and is writing a temporary folder to the storage location. That is successfully accomplished. So there does not appear to be a typical permission issue. But the next event is not described as a move but a rename of that folder. Specifically the process is

High Resolution Date & Time: 3/23/2018 12:36:08.3121537 PM
Event Class: File System
Operation: SetRenameInformationFile
Result: ACCESS DENIED
Path: C:\Users\Matthew Kostura\Documents\Zotero\storage\tmp-jn0ex7
TID: 21380
Duration: 0.0003793
ReplaceIfExists: True
FileName: C:\Users\Matthew Kostura\Documents\Zotero\storage\XVTXVAS8

There is no security program process or thread associated with any of the operations that I can see. I believe the error I am seeing is a failed interaction of the Windows Explorer file system and Zotero.

The temp file/folder is not present.

The process monitor I am using provides fairly extensive tracking and so far I have not noted another process especially a security program that is accessing the file.. But I could be missing something.

Problem isolated. After a fairly involved diagnosis, it did indeed come down to a piece of security software, but not my primary firewall, antivirus etc. I also make use of a program called MalwareBytes, a piece of software for preventing web based attacks. Zotero was set as an exclusion for the software but for some reason the software still blocks the connector from working.

Thanks and you are welcome. I found the link by running the machine in safe mode. When I started to look at my security software that is what came up. There is no solution in the settings in MalwareBytes. I even excluded the offending storage folder from scanning and that still did not help. The real time protection must be turned off or the program cannot be loaded....or as I have done uninstalled from the system.