A quick update: I was able to reproduce this. It is happening when an autocomplete becomes non-relevant, and the error is: "Reference to non-existing model id: autocomplete-model", autocomplete.xbl line 256, which corresponds to a:
<xf:setvalue ev:observer="external-value" ev:event="xforms-disabled" ref="$local/relevant" id="xf-599">
I'll have to investigate this further, and will let you know what I find.
Alex
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