FWIW, I investigated this as much as I could...all I could find is that there is some sort of difference in index files between versions. Once a user has been 'converted' to 2.3.11.3 index files, the error went away.

On the other hand I was told that index file format hasn't changed in a long time, so I really don't know what to think.

On Thursday, 15/10/2020 at 09:59 Stuart Henderson wrote:
On 2020-09-07, Scott Q. <qmail@top-consulting.net> wrote:
>
> Not sure if I mentioned it but I'm on FreeBSD too. I wonder if any
> of the patches FreeBSD applies automatically is causing this. I looked
> through them but couldn't find anything obvious that might cause this

FWIW also seen with 2.3.11.3 on OpenBSD, the only patches to code in the
OpenBSD port for this are unrelated.