Here is how I solved this problem.
So this morning I got this error on my Evolution mail, the first thing I tried was to restart evolution as this normally would solve a lock problem, but in this case it didn't work, evolution kept complaining about the lock and I was unable to access one of my mail folders while the other ones worked.
Then as I normally would do I searched google in hope someone else have had a similar problem and found a solution on this, but was not able to find any answers that solved this problem only suggestions, but none of them worked.
I gave up on google and looked in the .evolution directory starting with
$ find ~/.evolution/ -name *.lock
and found a
So I shutdown evolution.
$ evolution --force-shutdown
and removed the .lock file.
$ mv ~/.evolution/
and started up evolution again, then did no longer get the error message and my folder was yet again accessible.
This solved it for me.