did you edited your sandbox config ?
if not….
befor starting make a backup of your files….
look for <Mods /> and replace this to...
<Mods>
<ModItem>
<Name>WORKSHOP-ID.sbm</Name>
<PublishedFileId>WORKSHOP-ID</PublishedFileId>
</ModItem>
<Mods/>
so shud look like
<Mods>
<ModItem>
<Name>123456789.sbm</Name>
<PublishedFileId>123456789</PublishedFileId>
</ModItem>
<Mods/>
if you wana add more mods …..
<Mods>
<ModItem>
<Name>123456789.sbm</Name>
<PublishedFileId>123456789</PublishedFileId>
</ModItem>
<ModItem>
<Name>987654321.sbm</Name>
<PublishedFileId>987654321</PublishedFileId>
</ModItem>
<Mods/>
good luck