I ran into the error when compiling the same files on a desktop and a laptop. I run XeTeX as part of MikTeX 2.9.
The MWE test2.tex is:
\documentclass{article}
\usepackage{lipsum}
\makeindex
\begin{filecontents*}{test2.xdy}
(define-location-class "arabic-page-numbers"
("arabic-numbers") :min-range-length 1)
\end{filecontents*}
\begin{document}
\index{law}
\lipsum[1]
\index{law|seealso{system}}
\end{document}
and the xindy log on the laptop is:
Command Line: C:\Users\me\Documents\localtexmf\miktex\bin\texindy.bat "test2.idx" -M "test2.xdy"
Startup Folder: C:\Users\me\Documents\Law\Brunoy
Opening logfile "nul" (done)
Reading indexstyle...
Loading module "FZjbLHpwTY"...
Loading module "lang/general/latin9-lang.xdy"...
Loading module "lang/general/latin9.xdy"...
Finished loading module "lang/general/latin9.xdy".
Finished loading module "lang/general/latin9-lang.xdy".
Loading module "tex/inputenc/latin.xdy"...
Finished loading module "tex/inputenc/latin.xdy".
Loading module "texindy.xdy"...
Loading module "numeric-sort.xdy"...
Finished loading module "numeric-sort.xdy".
Loading module "latex.xdy"...
Loading module "tex.xdy"...
Finished loading module "tex.xdy".
Finished loading module "latex.xdy".
Loading module "latex-loc-fmts.xdy"...
Finished loading module "latex-loc-fmts.xdy".
Loading module "makeindex.xdy"...
Finished loading module "makeindex.xdy".
Loading module "latin-lettergroups.xdy"...
Finished loading module "latin-lettergroups.xdy".
Finished loading module "texindy.xdy".
Loading module "page-ranges.xdy"...
Finished loading module "page-ranges.xdy".
Loading module "word-order.xdy"...
Finished loading module "word-order.xdy".
Loading module "test2.xdy"...
WARNING: redefining location-reference-class `"arabic-page-numbers"' !
Finished loading module "test2.xdy".
Finished loading module "FZjbLHpwTY".
Finished reading indexstyle.
Finalizing indexstyle... (done)
Reading raw-index "4hBqChFmSb"...
Finished reading raw-index.
Processing index... [10%]
ERROR: SLOT-VALUE: The class #<STANDARD-CLASS LOCREF:CROSSREF-LOCATION-REFERENCE> has no slot named BASE:ORDNUMS
Removing the two lines in test2.xdy eliminates the problem. Commenting out any of the three lines between \begin{document} and \end{documents} also removes the problem.
While paring down my files to reach a MWE I got a variant of the error with "...no slot named BASE:TARGET".
No problems on the desktop, under the same version of MikTeX (2.9).