[Koha] Need help with Koha instance that refuses to do checkouts

[Chris Brown]

Dear Koha Community

Following up my post on 12 June I am repeating my call for help as Koha is
effectively unusable right now at one of our libraries.

We are running Koha 19.11 on Debian 9.11 (stretch). This supports 5 Koha
instances for some of the volunteer-run libraries in Sheffield. It has been
stable and working fine for many months.

Currently we have a serious problem with one of these instances. Any
attempt to check out a book hangs (never completes) and we see
/usr/share/koha/intranet/cgi-bin/circ/circulation.pl consuming most of the
CPU cycles (stuck in a loop perhaps?). On one occasion I noticed it was
renew.pl consuming all the CPU.

I have tried disabling plack for this instance, and have rebuilt the zebra
index, but the problem persists. There is nothing obvious reported in the
log files, though I may not be looking in the right places. I cannot see
any obvious problems with the configuration, though again I might be
looking at the wrong things. The other 4 instances are working OK.

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

Does anyone know how to turn on tracing of MySQL transactions? I was
wondering if looking at the activity at the database end might help.

Thanks and Best Regards,

Chris Brown (Voluntary Libraries in Sheffield)