New Messages go only to Local Folder; Account Inbox is bypassed and has no F5 function

In the last couple days, I am not receiving messages in my email account inbox (att.net/yahoo). Messages are going directly to the local inbox folder. Windows 10. When I search for new emails in the local folder, they are tagged with a gold diamond, and my option is to "Move to inbox again," which does move the messages back to the main inbox. But, "again" implies that the message arrived in the account folder, but got automatically passed to local folder. On or two occasions, I noticed an very fast dowloading to the inbox, which disappears immediately. And, I get no response in my email account when hitting F5. F5 works for the local folder, and other accounts, and Shift-F5 (getting messages for all accounts) updates the others, but not my main email account. The issue is not with att.net/yahoo. I have checked all my settings and server settings in Thunderbird, done the folder repairs several times, closed and reopened Thunderbird many times, deleted Inbox.MSF, etc., but nothing works. The issue seems to be specific to Thunderbird. Any ideas?