Early this morning I updated from 4.3.0-2125 to 4.3.1-2134. I didn't notice any failure to retrieve email at that time. At about 2:30PM I noted email was showing in the folder view of Thunderbird that there were unread email. I tried to sort by read status flag thinking that perhaps an email slipped, but neither top nor bottom revealed any unread email. I attempted to compact followed by folder repair from the properties menu of Thunderbird. The email folder suddenly went empty and could not be recovered. I reverted back to the previous version of DavMail and the behavior corrected without any further interaction.
I believe 4.3.1-2134 introduced this bug. I'm willing to submit logs if I'm allowed to make them anonymous in some way and that the testing is non-destructive.