It appears this is an IMAP account and you have drafts stored in the Drafts folder on the mail server (Copies & Folders in Account Settings). Does it make any difference if you change the target to Drafts on Local Folders? Unless you need access to drafts on other computers, it's better to keep them in Local Folders.

Thank you sfhowes. It's actually a POP3 account, but your reply guided me to the Copies and Folders under Account Settings, which I hadn't found on my own. I've just changed the selection to "Other," although it still seems to point to the same location.

In that case, it could be a file access error instead of a network error. Can't think what could cause that for a POP account, except possibly a 3rd-party security program interfering with autosave to the drafts folder.

If this happens with other operations, like saving sent copies, check that your security/AV isn't acting on the TB profile folder, which is the recommended setting anyway.

The error message does say there's a file access error. I'll recheck my AV software.

Meanwhile, it seems that manually clicking the "Save" button when the error message pops up resets some pointer to the right place. Once I've done a manual Save, the error seems to stop arising.

Sometimes, switching a setting to a different folder, and then switching back to the original setting, eliminates the error. So, you could try creating a temporary folder under Local Folders, and set drafts to store there, then restart TB and switch it back to Drafts on Local Folders, or whatever it was before.