Access Denied message, but only for the first request to edit a WebDAV PDF?

Has anyone had this issue?

We have a WebDAV editing capability, so users can “one click” edit a PDF from the browser.

For some reason, the first time they click to edit (first time since restarting their PC) Adobe Reader shows an Access Denied message.

If the user then closes Adobe and clicks edit again, the second edit always works.

This is consistently happening. We have tried adding the WebDAV URL to trusted sites, added it to Privileged Locations, etc. We have even inspected the protected mode log, but I’m not sure how accurate that is, because even the second successful edit shows many “STATUS_ACCESS_DENIED” entries.

Here’s a snippet of a failing log:

[06:11/14:44:39] real path: \??\UNC\redacted.com\davwwwroot\token\1980306\redacted.pdf
[06:11/14:44:39] Consider modifying policy using these policy rules: FILES_ALLOW_ANY or FILES_ALLOW_DIR_ANY
[06:11/14:44:39] NtOpenFile: STATUS_ACCESS_DENIED
[06:11/14:44:39] real path: \??\UNC\redacted.com\davwwwroot\token\1980306\redacted.pdf
[06:11/14:44:39] Consider modifying policy using these policy rules: FILES_ALLOW_ANY or FILES_ALLOW_DIR_ANY
[06:11/14:44:39] NtOpenFile: STATUS_ACCESS_DENIED
[06:11/14:44:39] real path: \??\UNC\redacted.com\davwwwroot\token\1980306\redacted.pdf
[06:11/14:44:39] Consider modifying policy using these policy rules: FILES_ALLOW_ANY or FILES_ALLOW_DIR_ANY
[06:11/14:44:39] NtCreateFile: STATUS_ACCESS_DENIED
[06:11/14:44:39] real path: \??\UNC\redacted.com\davwwwroot\token\1980306\redacted.pdf\
[06:11/14:44:39] Consider modifying policy using these policy rules: FILES_ALLOW_ANY or FILES_ALLOW_DIR_ANY
[06:11/14:44:39] NtOpenFile: STATUS_ACCESS_DENIED
[06:11/14:44:39] real path: \??\UNC\redacted.com\davwwwroot\token\1980306
[06:11/14:44:39] Consider modifying policy using these policy rules: FILES_ALLOW_ANY or FILES_ALLOW_DIR_ANY
[06:11/14:44:39] NtOpenFile: STATUS_ACCESS_DENIED