I believe that bacula does not work anymore with curlftpfs (version 0.9.2).
I have found several reports which describe a similar behavior.
https://bugs.launchpad.net/ubuntu/+source/curlftpfs/+bug/367091
http://sourceforge.net/projects/curlftpfs/forums/forum/542750/topic/3295831

Finally I "found" the release notes of curlftpfs 0.9.2:
http://sourceforge.net/project/shownotes.php?release_id=602461
"Be aware that some applications might not be able to "save" files on 
curlftpfs from 0.9.2 on, because we don't support open(read+write) or 
open(write) and seek anymore."

I have written a small program to test this functionality. You can use 
it to verify your environment.
#include <stdio.h>
#include <string.h>

  int main(void) {
    //FILE *fp = fopen("/home/bacula/test","w+b");
    //works fine (without curlftpfs)
    //FILE *fp = fopen("/home/bacula/archive/Daten/Backup/test","a+b");
    //create flags: 0x442 /Daten/Backup/test 0100644 umask=0000
    //ftpfs: operation ftpfs_open failed because Operation not supported
    //FILE *fp = fopen("/home/bacula/archive/Daten/Backup/test","a+");
    //create flags: 0x442 /Daten/Backup/test 0100644 umask=0000
    //ftpfs: operation ftpfs_open failed because Operation not supported
    //FILE *fp = fopen("/home/bacula/archive/Daten/Backup/test","w+");
    //create flags: 0x242 /Daten/Backup/test 0100644 umask=0000
    //ftpfs: operation ftpfs_open failed because Operation not supported
    FILE *fp = fopen("/home/bacula/archive/Daten/Backup/test","w+b");
    //create flags: 0x242 /Daten/Backup/test 0100644 umask=0000
    //ftpfs: operation ftpfs_open failed because Operation not supported

    fprintf(stdout, "I try to open file\n");
    if(fp==NULL)
    {
      fprintf(stdout,"Error: can't open file.\n");
      return 1;
    }
    else {
      char str[40];
      int i;

      strcpy(str,"somecharacters");
      printf("File opened successfully. Writing....\n\n");
      for (i=0;i<8;i++) {
        fputc(str[i],fp);
      }
    }
    fclose(fp);
    return 0;
  }

I would recommend that the documentation of bacula should mention this 
incompatibility.
Using a ftp service you have to find an alternative (which one?)

Br, Karsten


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