Error with closed appender

[Bonio Lopez <bo...@gmx.de>]

Dear All,
I have one appender for the whole application it is initialized on the start
and never explicitly closed. 
Sporadically, on the different places in one module I get following error:
[3464] log4net:ERROR [FileAppender] Attempted to append to closed appender
named []. 
I tried out minimal lock, exclusive lock but with the same result. As I have
written, the logger stops to work on different places even if I run program
with exactly the same parameters.
Please help me to debug that. I am completely lost after day and days of
struggling :)
With best thanks,
Boni