[Evolution] Evolution forces conversion from mbox to maildir

[Martin Wagner <mwrsa web de>]

Hey everybody!
I can't imagine that I am the first one to run into this. Still I found
no solution, neither did I get any kind of response from the Linux Mint
forums, so I am asking here.

System info: Mint 19.2 Mate, all updates installed, Kernel latest 5.3;
Evolution V.3.28.5-0ubuntu0.18.04.1 (which is the latest available for
my Mint 19.2 ootb)

As of today (written on Dec 5th, 2019), after an update probably,
Evolution refused to start without converting my email storage from
mbox to maildir. Since I had no choice anyway, I agreed and started the
conversion (I have a backup as of yesterday, luckily!) and waited quite
a bit. Yes, it's a single thread conversion, and we are talking about a
massive archive here.

Once it started, then, I found that not a single mail had been
converted. All I got to see were the latest mails received today. Also
the local folder containing the backup of the former mbox archive was
entirely inaccessible, not a single mail I tried to open showed up,
just an error message (roughly translated from German: Failed to open
folder, folder cur/<foldername> doesn't exist.).

Well I employed my backup and got all my working mbox files back. While
using the manual import of Evolution on over 7000 single files is
simply impossible, I searched the web for a solution and stumbled over
mb2md. Gave it a try, of course. The outcome was just as great as
Evolution's conversion itself: The few files it did not skip because of
the leading dot, it did not recognize as valid mbox file and skipped
them as well (The manual import of Evolution recognizes them though and
works correctly on a single file.). It seems to me like Evolution used
an own integrated mb2md algorithm, because that would explain the
similar outcome.

Enough speculation. Do you have any solution for a working batch
conversion please? Thanx.

Cheers, Martin