The best way to tackle this issue would be from a Linux-PC, may that be running a Live-CD or -USB or even an installed system. You need Backupmenu installed to the N900.

First repair the MyDocs Partition via fsck
Second take a full backup (YES even from the damaged optfs a backup! )
Third try to repair the file system of optfs with fsck, testdisk or gparted.

Do you have backupmenu installed at all?
Do you have an external microSD card or USB-stick to clone optfs to to 'repair' it there to not fumble with the original too much?