How do I figure out why systemctl service "systemd-modules-load" fails?

I too had this issue. I was able to solve it by following the instructions on the Arch Linux systemd wiki page. Here is a summary of what I did :

1. Lets find the systemd services which fail to start

   $ systemctl --failed  
   ------------------------------------------------------------------------
   systemd-modules-load.service   loaded failed failed  Load Kernel Modules
   

2. Ok, we found a problem with systemd-modules-load service. We want to know more.

   $ systemctl status systemd-modules-load  
   ------------------------------------------------------------------------
   systemd-modules-load.service - Load Kernel Modules      
      Loaded: loaded (/usr/lib/systemd/system/systemd-modules-load.service; static)     
      Active: failed (Result: exit-code) since So 2013-08-25 11:48:13 CEST; 32s ago  
        Docs: man:systemd-modules-load.service(8).  
              man:modules-load.d(5)
     Process: 15630 ExecStart=/usr/lib/systemd/systemd-modules-load (code=exited, status=1/FAILURE)
   

   If the Process ID is not listed, just restart the failed service with

   $ systemctl restart systemd-modules-load
   

3. Now we have the process id (PID) to investigate this error in depth. Enter the following command with the current Process ID (here: 15630):

   $ journalctl _PID=15630
   ----------------------------------------------------------------------
   -- Logs begin at Sa 2013-05-25 10:31:12 CEST, end at So 2013-08-25 11:51:17 CEST. --
   Aug 25 11:48:13 mypc systemd-modules-load[15630]: Failed to find module 'blacklist usblp'
   Aug 25 11:48:13 mypc systemd-modules-load[15630]: Failed to find module 'install usblp /bin/false'
   

4. We see that some of the kernel module configs have wrong settings. Therefore we have a look at these settings in /etc/modules-load.d/

   $ ls -Al /etc/modules-load.d/
   ----------------------------------------------------------------------
   ...  
   -rw-r--r--   1 root root    79  1. Dez 2012  blacklist.conf  
   -rw-r--r--   1 root root     1  2. Mär 14:30 encrypt.conf  
   -rw-r--r--   1 root root     3  5. Dez 2012  printing.conf  
   -rw-r--r--   1 root root     6 14. Jul 11:01 realtek.conf  
   -rw-r--r--   1 root root    65  2. Jun 23:01 virtualbox.conf  
   ...  
   

5. The Failed to find module 'blacklist usblp' error message might be related to a wrong setting inside of blacklist.conf. Lets deactivate it with inserting a trailing # before each option we found via step 3:

   /etc/modules-load.d/blacklist.conf  
   ----------------------------------------------------------------------
   # blacklist usblp  
   # install usblp /bin/false  
   

6. Now, try to start systemd-modules-load:

   $ systemctl restart systemd-modules-load  
   

   If it was successful, this should not prompt anything. If you see any error, go back to step 3 and use the new PID for solving the errors left.

   If everything is ok, you can verify that the service was started successfully with:

   $ systemctl status systemd-modules-load
   ----------------------------------------------------------------------
   systemd-modules-load.service - Load Kernel Modules
      Loaded: loaded (/usr/lib/systemd/system/systemd-modules-load.service; static)
      Active: active (exited) since So 2013-08-25 12:22:31 CEST; 34s ago
        Docs: man:systemd-modules-load.service(8)
              man:modules-load.d(5)
    Process: 19005 ExecStart=/usr/lib/systemd/systemd-modules-load (code=exited, status=0/SUCCESS)
   Aug 25 12:22:31 mypc systemd[1]: Started Load Kernel Modules.