This is the error?

It's a known issue, and not space related. Unknown how soon it will be resolved.

What version are you using? Do you still see the issue? Is this happening only on an empty folder, i.e. a folder which has no messages?

138.0.2 (64-bit)

It's my main email inbox. Happens all freaking day.

I upgraded to 139. YUCK! What a freaking mess this is...and still getting the storage errors. The layout is horrific. Who thought this up? Plus, PDFs are now showing as Winmail.dat files and NOT PDFs. Just terrible.

I upgraded to 139. YUCK! What a freaking mess this is...and 1. still getting the storage errors. 2. The layout is horrific. Who thought this up? 3. Plus, PDFs are now showing as Winmail.dat files and NOT PDFs. Just terrible.

Doug. Thanks for posting this detailed information.

1. Yes, that issue would continue. 139 also doesn't have a fix for the compact warnings.

2. There is a bug in 139 that will soon have a fix. You can resolve this by doing the two steps at https://support.mozilla.org/en-US/kb/getting-started-thunderbird-main-window-supernova#w_classic-view-layout-with-table-view

3. Do you have https://addons.thunderbird.net/en-US/thunderbird/addon/lookout-fix-version/ installed? That would likely cause this issue. The author has not yet updated it to be compatible with 139.

No, I do not have that addon. The Inbox in table view looks correct again but the attachments are still messed up. I have to go to a browser to download the file(s).

The winmail.dat file is an indicator of the email having been sent in a Microsoft proprietary encoded email format that uses what Microsoft refers to a TNEF format. Only Microsoft Outlook/Exchange server (which generates this rubbish) and a very few other application understand the format. Since as long as I can remember, Thunderbird has used the lookout addon to convert these proprietary mails into something that makes sense. Thunderbird has never had this ability natively. If you previously had the ability to see the contents of TNEF message and no longer can you have been using an addon to do so. Most probably the lookout one which as yet does not support version 139, the last version supported by the addon being 138 as Wayne said. See https://github.com/TB-throwback/LookOut-fix-version/issues/124

If it offends you that your correspondents are sending you proprietary mail in the expectation that you will be using Outlook to read what they send please contact them and explain your annoyance. There is no point expressing it here because no one here can affect the format your correspondents choose to send you mail.

Some extended information on TNEF/Winmail.dat;

The library of congress has this to say about TNEF. https://www.loc.gov/preservation/digital/formats/fdd/fdd000485.shtml

Microsoft have this to offer https://learn.microsoft.com/en-us/outlook/troubleshoot/message-body/how-tnef-affects-email-messages and https://learn.microsoft.com/en-us/exchange/mail-flow/content-conversion/tnef-conversion

Inbox disappearing is possibly https://bugzilla.mozilla.org/show_bug.cgi?id=1952311. Also still being worked on.

Combining https://support.mozilla.org/en-US/questions/1516878 and https://support.mozilla.org/en-US/questions/1513607 into here

I have found that just clicking on the account and waiting a couple seconds and then going back to the Inbox brings the messages back.