I saw this error message on a Cisco ASA 5540 today:

%ASA-3-210005: LU allocate connection failed

According to Cisco, this error means the standby unit was not able to create a new connection for stateful failover. The recommendation is to verify that the system has enough RAM.

OK...

# show memory
Free memory:       987969608 bytes (92%)
Used memory:        85772216 bytes ( 8%)
-------------     ----------------
Total memory:     1073741824 bytes (100%)

As you can see, this system has a lot of free memory? So that's obviously not the issue.

I thought perhaps the problem was related to some sort of communication problem between the standby and active devices, but that wasn't the case either. I did a little in the Cisco Bug Toolkit and found CSCsh80889: LU allocate connection failed msg due to failed VPN flow replication.

The fix is to upgrade to a more recent version of Cisco ASA software.