Marc,
I'm a bit confused by the description, you say at the small site everything works fine but then later you say that ping and file shares don't work? Can you clarify what exactly works (only internet?) and what doesn't (all access to the main site?) ?
In any case, from that error you quote, this sounds like a routing issue: note that it says "src inside: dst inside:" so it thinks the destination is on the inside (while it should be on the outside, across the vpn tunnel).
If you'd like some help troubleshooting this further, we'll need more details - would you mind posting your configs and the full syslog message?
hth
Herbert