my profile was about 10GB in size

I suppose a large part of the 10 GB would be your Inbox. Accumulating messages in Inbox isn't best practice in will ultimately cause problems. Particularly if your anti-virus software is trying to scan the Inbox file every time Thunderbird is trying to write to it.

So the problem you're seeing may be a combination of different issues, including something on the Yahoo server.

It's recommended to break up your giant Inbox into smaller pieces on the server. Don't let your anti-virus software scan your Thunderbird profile folder. Follow best practice and keep your Inbox as tidy as possible. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

Thanks for replying. So you are saying I should move the messages out of the inbox and into a new folder and that may solve the problem? It is interesting you mention that, because the error only ever mentions the Inbox folder.

I read that earlier about the anti-virus, so I disabled Avast, but still get the same error.

So would you recommend I create a new folder in Yahoo and move everything from Inbox to it? If you think that will make it work, I'll give it a try...but since I don't really plan to use Thunderbird to write messages or anything...I just want it to backup all my emails locally, I don't want to get too carried away re-configuring my Yahoo account because the Yahoo webmail, which is how I always access the account, works okay the way it is.

So should I create a new folder or maybe a few new folders and move my Inbox to it? Do you think that will fix it and then it will download all my emails so I can have a local copy?

So you are saying I should move the messages out of the inbox and into a new folder and that may solve the problem?

Not exactly. The idea is more to create smaller chunks than one giant folder, e.g. an archive with one folder per year.

I disabled Avast, but still get the same error.

Try to start Windows in safe mode with networking enabled. - Win10 http://windows.microsoft.com/en-us/windows-10/change-startup-settings-in-windows-10#v1h=tab01 - Win8 http://windows.microsoft.com/en-us/windows-8/windows-startup-settings-safe-mode# - Win7 http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

Wrt. Avast see https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVAST

So I went ahead and started Windows in safe mode. I then started Thunderbird in safe mode. I still got the exact same error message.

So I decided to try to split the inbox into another folder to see if making a smaller folder would help. I put about 1/3 of the inbox into a second folder...and interestingly, I got the same UID Fetch error, but this time it also listed the name of the new folder I had just put the messages into.

Maybe that folder was still too 'big'? Although I really don't get why I should need to break the inbox down into a bunch of smaller folders? Using webmail I have no problems keeping them all in the inbox, so why would Thunderbird have an issue with them all in 1 folder vs in 10 folders?

At this point I'm not sure if this problem is occurring because of Thunderbird or Yahoo? But I think maybe what I'll do is I'll try to download another email client like Zimbra or whatnot and see if it works or if I get an error without another program as well.

Just to provide an update in case anyone else has this problem and comes across this topic in the future...

I went ahead and installed Zimbra Desktop. I setup my account and it downloaded all my messages without a problem...not a single error message at all.

I spent more than 2 weeks with Thunderbird and got the same error over and over again...I installed Zimbra and had everything downloaded and synced flawlessly. Not sure why that is or why Thunderbird had problems...but just in case anyone else has the same problem, this might help save them some time and frustration.