Odd one this .

I have a 2830 @ HQ Firmware 3.6.7.2_sb_2471201 and a 2820 Firmware 3.3.7.5_232201 .

The LAN to LAN (PPTP) comes up and links for 3 to 8 secs ... no connection , no data xfer..

Create a Smart VPN software link from 2820 remote site ... connects with data flow and resolution .

Have swapped routers at both and and 2820 will connect to other sites . BT Fibre support refusing to diagnose line or Fibre router as they can VPN in with software .
At one stage had 2 x 2820 , same f/w at both sides same outcome ... any help appreciated .

Check one of your additional LANS subnet on the 2830 is not set to the same as the remote routers subnet. Even if you're only using one subnet on the 2830, change the unused subnets to something else.