Help with failures on HEAD

Hi,

Last Saturday morning, builds of HEAD on my animal mantid[1] spontaneously started failing;
see [2] for an example. The log shown there is

Missing checked out branch bf_HEAD:
* [32mbf_HEAD[m
bf_REL_11_STABLE[m
bf_REL_12_STABLE[m
bf_REL_13_STABLE[m
bf_REL_14_STABLE[m
bf_REL_15_STABLE[m
master[m

and the emails I get from cron contain multiple copies of the line

Use of uninitialized value $commit in concatenation (.) or string at /u1/tac/build-farm-16/PGBuild/SCM.pm line 1381.

I tried removing buildroot/HEAD, and “./run_build.pl --nosend --nostatus --verbose HEAD” succeeded.
However, the next time cron ran the builds, all branches except REL_11_STABLE started failing the
same way as HEAD. I then removed buildroot entirely, which fixed all the branches except HEAD.

Mantid is using v16 of the build farm client and is running CentOS 7 with perl 5.16.3.

Any advice on how to fix this would be greatly appreciated.

-- todd

[1] https://buildfarm.postgresql.org/cgi-bin/show_status.pl?member=mantid
[2] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=mantid&dt=2023-02-04%2008%3A07%3A02