Hans,
after the fix in current latest (2021.03.09 11:42), I get the following
error when using \in[\xmlrefatt{#1}{href}] in Windows (Linux has no
problems).
The error message reads:
fatal lua error:
run callback [6]: bad argument #2 to 'format' (number expected, got nil)
stack traceback:
[C]: in function 'string.format'
?: in field '?'
...ext/tex/texmf-context/tex/context/base/mkxl/luat-log.lmt:178: in
upvalue 'report'
...ext/tex/texmf-context/tex/context/base/mkxl/luat-log.lmt:391: in
upvalue 'report'
[string "