What kind of accounts are being filtered, POP or IMAP, and what is the format of the filters, i.e. filter rules and actions? The latest bug report I could find is here.

http://kb.mozillazine.org/Filters_(Thunderbird)

I am experiencing this issue in 68.4.1 (on Windows 10 Home 1909), with all filters, across multiple accounts, all using POP3, and four different providers.

The filters concerned are on different properties (Subject, From, To, mainly), and all move messages to Local Folders. All work exactly as expected when run manually (so the issue is not in the terms of the filters themselves), but do not run at all while mail is being received. "Manually run", "Getting new mail", and "After sending" are all checked.

The log records error code 0x80520012 each time.

TLockyer said

I am experiencing this issue in 68.4.1 (on Windows 10 Home 1909), with all filters, across multiple accounts, all using POP3, and four different providers. The filters concerned are on different properties (Subject, From, To, mainly), and all move messages to Local Folders. All work exactly as expected when run manually (so the issue is not in the terms of the filters themselves), but do not run at all while mail is being received. "Manually run", "Getting new mail", and "After sending" are all checked. The log records error code 0x80520012 each time.

This old post for Firefox points to extensions as the cause for a similar error. Does the same error happen in TB if you run in safe mode? Help/Restart with Add-ons Disabled. It could also be an antivirus issue, if you have it scanning the profile folder. Checking for mail and scanning it might conflict in a filter, but not when it's run manually.

sfhowes said

Does the same error happen in TB if you run in safe mode? Help/Restart with Add-ons Disabled.

I tried that, and the same happened. (I have three extensions, BTW, the preinstalled Lightning, ImportExportTools NG, and Click and Send.)

It could also be an antivirus issue, if you have it scanning the profile folder. Checking for mail and scanning it might conflict in a filter, but not when it's run manually.

I will check that.

One detail I should have mentioned: mail is set to manual check; not startup or automatic.

I wrote

One detail I should have mentioned: mail is set to manual check; not startup or automatic.

I have run a couple of tests, and it seems this might have been the issue: with all automatic mail checking options checked, the filters run automatically. It should not work this way, but apparently it does.

In the Filter Rules window, there are choices to Apply filter when: 'Getting New Mail' and 'Manually Run', so if the first one is not selected, the filter won't run if automatic or startup fetching is disabled, and I would expect it also not to run if fetching mail is done manually.

sfhowes said

In the Filter Rules window, there are choices to Apply filter when: 'Getting New Mail' and 'Manually Run', so if the first one is not selected, the filter won't run if automatic or startup fetching is disabled, and I would expect it also not to run if fetching mail is done manually.

As mentioned in my first comment, I did have "Manually Run", "Getting New Mail", and "After Sending" checked for all mail filters.

What I did not have checked (because I would have preferred to initiate mail receipt manually) were "Check for new messages at startup", "Check for new messages every [10] minutes", and "Automatically download new messages", all under the "Server Settings" for the affected accounts.

It seems to me that it should be possible to have filters run automatically when manually getting messages, but apparently that does not happen.

One further detail, and again it is not obvious why it works this way. In the filter settings, against "Getting New Mail" one has the option to filter before or after Junk Classification. The filters work on manual execution regardless of whether one selects before or after, but I have found after testing that if one selects "before", then the filter does not work automatically.