Those are good signs, especially with you being able to sync from other devices that your setup is correct and your data itself isn’t a problem. If at any time you would rather call support, please do so and only try what you are comfortable attempting.
Questions and Details
Can you check on the Mac that is not syncing which OmniFocus version is installed? OmniFocus menu, About OmniFocus, and the exact number should show there. If the version is 2.7.1, that is up to date and has the keychain fix for encryption that solves an issue like this with some WebDAV servers. If you have 2.7.1 just ignore this, otherwise update OmniFocus. Did this begin after you updated OmniFocus? Or has it synced on this version with success?
On your MacBook that is syncing, go to OmniFocus menu, Preferences, Synchronization and click Show Sync Details. Click Encryption. If it says Encryption is active, does it say the passwords are linked or distinct? Or that Encryption isn’t active? I ask because the suggestions would be different depending on these settings. If you’d rather not share that in public it is good information for support to know. If Encryption is disabled, that detail is good to know as well.
Since it was syncing before, what changed around when it failed? If you go to the Mac where syncing is failing, under the OmniFocus menu, check Preference, Synchronization, Show Sync Details>Log. Is there anything that happened right before sync failed in that log? It may help support to have a copy of that log, especially right before and after it stopped being able to sync. The reasons for the Sync can be useful in trying to determine why it stopped working.
What does the console say? On the non-syncing Mac, type Console into Spotlight, press return. Attempt to sync. What is printed in the Console? There may be a more detailed error that will help us determine the problem. If it matches the error you’ve already shared, that is also good to know.
Is there a firewall or any anti-virus software running on this Mac that isn’t syncing? If so, try adding an exception in that software temporarily for OmniFocus and attempt to sync again.
Which version of MacOS (or macOS as the case may be) are you using? You said you recently got a security update previously. Any changes that could be relevant here? I will try the same update here to see if I can reproduce the issue if we aren’t able to resolve this now.
You could try going to the Server WebDAV folder to the OmniFocus.ofocus file, rename it temporarily or delete it if you have another backup, then Sync from one of the syncing devices that are working. See if this allows you to sync from the Mac.
If still unable to sync on that same Mac as before, type Keychain Access into your Mac Spotlight and press Return. Find just the key for your WebDAV account on the Mac and delete it. Try to Sync, which should ask you for your password. Enter the password and see if you are able to sync. If you are not able to sync, try step 3.
With OmniFocus on Mac launched, Control click on the icon at the top of the window, Click the OmniFocus folder from the menu that appears there, and rename OmniFocus.ofocus to “Backup.ofocus”. Now quit and relaunch OmniFocus where you’ll set up syncing again. You’ll enter your WebDAV info to point OmniFocus to the right place and set up the preference again. Now try to sync. If still unable to sync try step 4.
Make a new folder on your Server and try to sync to the empty folder from just the Mac that can’t sync. I’m only suggesting this because the folder permissions changing helped before. If the new folder gets syncing to work, go to the MacBook that is syncing, enter in that path in preferences but do not sync, then replace server database from the file menu to get your newest info. If this works, there is a problem with either the folder permissions or something about the file inside that is the cause.
If encryption is on and this started after you migrated your database to the new format, you may want to try to restore a pre-migration backup on the device you first migrated on. Before you try this please export an OmniFocus Backup locally to be sure none of your data is lost. After syncing is working, open up this backup and use copy/paste to bring in anything added since the backup. Under the File Menu, go to Backups on one of the syncing devices and search for the backup named “Before Migrating Database” in the title. Restore that backup. If you do this, you will be notified on each device when you sync next that Encryption isn’t used. If this works to get all devices syncing, make sure that you are on version 2.7.1 and sync all devices before migrating forward. If the pre-Migration database can sync, there may be an issue with Encryption on this device. If this suggestions works I can send you some further options on how to proceed.
On the Mac that can’t sync: Create a new user account and Install OmniFocus to it. If you can sync from a new user account, but not your account, something may be wrong in your User Settings on the Mac or something unique about that user profile causing the issue.
I hope at least one idea can help you. I know the list is really long, but it is everything I would try if this were my machine not syncing. Please skip anything you are uncomfortable with and rely on support if you’d prefer a more specific suggestion. If one of these ideas gets you syncing again, please let us know which step so that someone else in the same situation can try that step.