Audioengine segfault libardour.so.3

My Ardour 6.9 on Jack has suddenly started crashing with the following error in journal:
kernel: audioengine[3370]: segfault at 229789b2c0 ip 00007f185d7b7e1b sp 00007f1822318a30 error 4 in libardour.so.3[7f185cfd3000+c58000]
It happens even on a clean project with no plugins loaded. How do I troubleshoot this issue?

To do this correctly, you need a debug build.

And 6.9 is not available as a debug build?

We do not make debug builds routinely available. If the crash occurs in “safe mode” (all plugins disabled; available on the session selector dialog during startup), email me and I’ll get you a copy.