The simple answer is that when you created the MXD, you may not have checked store relative path, which is situated on the file manu in ArcMap. Open your mxd in ArcMap, check that it is/isn't set and republish it.
If it is set to storing the relative path, then the only real alternative to the error is that the MXD, or data, is situated in an area in which the SOC account has no permissions. So if the first one doesn't work, then go back and ensure that the folder the mxd is in, and all data, is shared by the SOC/SOM accounts.
When you add data to a map, the pathname to the data is stored in the map. When you open your map, ArcMap locates the data it needs using these stored pathnames. If you plan to distribute your maps to others or if the location of your data has changed, you may need to change how your map references data so that when you save the map, others will not need to repair layers.