An h extension is the special extension that is run when a channel hangs up. Long running means running longer than the timeout for destroying the SIP dialogue.
If you are getting other symptoms of a deadlock, the message you quote is a symptom, not the problem.
When you say you cannot create SIP channels, do you meant that attempts to create them to any endpoint stop dead in their tracks, or does the problem only affect the endpoint in question here, and does the attempt end with an error?
For deadlocks, make sure you are running the latest supported sub-version of a supported version, then see https://wiki.asterisk.org/wiki/display/AST/Getting+a+Backtrace#GettingaBacktrace-GettingInformationForADeadlock