[Users] sometimes claws forget the read-status of messages

[c.buhtz at posteo.jp]

I am using Claws 3.13.12 on a Siduction Linux (Debian unstable) with
POP3/SMTP accounts.

Sometimes (but not exact reproducable) it forget the read status of
messages.
e.g. when I start claws INBOX is shown me with 2 unread meassages in
the left folder tree. But when I click on INBOX to show the messages in
it some of the older messages in it (sometimes all) turn to unread.

This problem comes from time to time but not so often. I am not able to
reproduce it for sure. That is why I am not sure what causes that
problem.

Is there a way to "reset" everything claws knows about the (un)read
status of its local stored messages? Is there a central file where this
info is stored and I can delete it?

For the background info:
I use the same Claws "instance" on two machines! This means I sync the
~/.claws-box and ~/.claws-mail folders between two machines. The last
years I used the Ubuntu Trusty claws version (quite old) for that.
I had the same problems there.
I don't use them as the same time! I switch between a desktop pc and a
laptop. When switching I start a sync process (with Unison) by myself.
This work really well. But only the thing with unread messages.