'bzr explore' unexpectedly throwing MS Visual C++ runtime error at startup

[DeeJay]

I'm not bug-reporting this in the first instance because I fear it is
a local issue.

WinXP, with bzr-2.1.0rc1-1-setup.exe installer has been running
happily for several days. This morning, on trying to run bzr-explorer,
I get a Microsoft Visual C++ runtime error. The console reports
"QVariant::load(QDataStream &s): type list unknown to QVariant."
Nothing is written in .bzr.log

I've tried uninstalling Bzr via Add/Remove programs, and then
re-installed from the  bzr-2.1.0rc1-1-setup.exe installer that was
saved on my system from the previous install. No improvement.

Other tools from the qbzr suite seem to run correctly, so it isn't
that the Qt support is completely dead.

Calm logic leads to the unpleasant conclusion that if I've removed and
re-installed bzr and friends then the problem must be elsewhere in my
system. But where and how/why has it got broken?

Any ideas, please?