Are you using Mobile 9.3.1 or 10? Is there anything that has changed with the data or schema in SDE since you created the mobile project?
I also attempted to have a two-way replica, mainly for quality control since our data is coming from a department that doesn't have much GIS experience. I had trouble making this work properly, so I reverted to a single SDE non-versioned, and I also turned off archiving. (we have a software backup that works as well as archiving)
If you have access for the SOC, your data has Global IDs and you are not trying to version the data, I would expect this to work. The posting of data from Mobile is a first come first serve affair, which is a reason I would much prefer the versioned data to work. Are you using the AGM_XXX fields to log your users?
It's possible this is a IIS issue. (Doubtful but it's worth a look)
Check this link: http://forums.arcgis.com/threads/12596-Can-t-save-mobile-project-to-IIS-web-server?p=72493#post72493
Scroll to the bottom of the post, and you might find something relevant to your web setup that could cause this error.