I hunted for two days after an index error in my book: """ structure > sectioning > chapter @ level 2 : 0.1 -> Sachregister tex error > tex error on line 95 in file ./prd_test.tex: The file ended when scanning an argument. {\startregisterentries {1}\startregisterentry {0}\registerentry {index}{}{21}{}{[[special cmd: lua call]]}\startregisterpages \registeronepage {index}{}{21}{2}{[[special cmd: lua call]]}\stopregisterpages \stopregisterentry \stopregisterentries \stopregistersection \stopregisteroutput \relax \stopmixedcolumns \else \strc_registers_place_indeed {index}\fi \endgroup \fi \stopchapter \par \par \par \par \par \stopproduct """ It boiled down to \index{\%} that used to work but doesn’t any more in current LMTX. Also, "context --purge" doesn’t delete the tuc file (don’t know if that changed), so the wrong entry stuck in there, and my code changes didn’t affect it. MWE: \starttext %\index{\%} \index[ ]{\backslash} \index[ ]{\percent} \index[ ]{\tex{}} \index[ ]{\tex{}\tex{}} \index[ ]{\tex{\%}} \page \completeindex \stoptext For my command index, \index[something]{\tex{something}} works, but is there a better way to put \\ in there than \index[ ]{\tex{}\tex{}} ? Hraban