There are 2 things happening there. Firstly, /dev/sdb1 (/opt) filled up and secondly /dev/sda2 (/swap) filled up.
For the first one, since you've already re-booted the box and run fsck on it, I don't think the problem is related to deleted files that have not been released. There's some info in the SLUG mailing list archives
about this. By default there is 5% (about 3.4GB in this case) of the operating system reserved for the super-user. Can you remove, even temporarily, a few more gigs of data and see if normal users can write to the file system again?
For the second one, I think your swap file is filling up the /dev/sda2 partition and the "E12: Error write in swap file" error occurs when it runs out of space. If you remove the swap file, change the partition to type swap, then re-create it as a swap partition then this should not happen.