L2L tunnel up, not passing traffic...all of a sudden

Question

I've had a tunnel in place on a 5505 to a remote network i don't control...so my troubleshooting there is limited.  But the tunnel has been in place for over a year without issue.  Suddenly it doesn't appear to be passing traffic.  But it is in at least one direction.  
Remote network:192.168.191.0/24
Local ASA side: 10.220.78.0/24
I had a constant ping started from 192.168.191.10 > 10.220.78.23
Which is a Windows server pinging a Windows workstation.
When i debug icmp on the ASA i get:
ICMP echo request from outside:192.168.191.10 to inside:10.220.78.23 ID=1 seq=2866 len=32
ICMP echo reply from inside:10.220.78.23 to outside:192.168.191.10 ID=1 seq=2866 len=32
Which confirms to me that the remote network is in fact traversing the tunnel and hitting the 10.220.78.23 device, which is in fact responding, and the reply is being sent out the ASA.
The tunnel negotiates and comes up any time I reset it, by all accounts it looks correct.
The problem is not limited to ICMP as I'm unable to net use or map drives, nor can 192.168.191.10 print to the printer at 10.220.78.20.
But once i saw the icmp trace output I pretty much figured it has to be on the remote end...so....
My question, can I absolutely infer from this that the issue resides on the remote end?

Answer 1

Some additional info.  Aside from the ping they have running from the remote network, which is shown in the above icmp trace, if i run packet tracer from the local network to the remote, tunnel's up/traffic is allowed.  Not a big surprise since the tunnel does negotiate and stay up.
I captured packets from the ASA and I can see the local 10.220.78.23 device sending the reply to 192.168.191.10.  Matching up with the icmp trace.
I had them run a packet capture on their firewall and confirmed, the ICMP requests from 192.168.191.10 are being encapsulated and sent on the tunnel.  Again confirmed in my mind since i see the requests on the ASA.  But they don't ever see the response.
There's no tcp adjust mss command on the ASA but there's this in the config:
ASA# sh run all sys
no sysopt connection timewait
sysopt connection tcpmss 1380
sysopt connection tcpmss minimum 0
sysopt connection permit-vpn
sysopt connection reclassify-vpn
no sysopt connection preserve-vpn-flows
no sysopt nodnsalias inbound
no sysopt nodnsalias outbound
no sysopt radius ignore-secret
no sysopt noproxyarp inside
no sysopt noproxyarp outside
Any other ideas?