It doesn't occur on closing MePo, it's just that you see the error has occurred when you close MePo
This normally indicates an important file is not accessible (readable, writeable). In this case it can be that mediaportal.xml is still be written to. I think a plugin (or a combination of plugins) trying to write their configuration to mediaportal.xml while mp itself tries to shutdown... The nearest matching process in your logfile is MPEI saving settings. But if this is the thing blocking the file, I don't know really...CLR20r3