Hi,
The error is issued because the search response data is modifed – I assume weglot tries to translate everything “on the fly” and also hooks into the request response. However the response data contains JSON structured information, which should not be changed, otherwise it is broken and can’t be decoded. JSON is normally used with ajax requests, so I am not exactly sure why would the translator try to change the XHR response data. I guess if the XHR response format is not exclicitly JSON then it will just automatically apply.
I believe we tried to solve this some time ago, and if I recall correctly, then the main bascially unsolvable issue with this plugin was, that it translated in real time during output. I belive the translations weren’t stored as separate post type objects, but additional data somewhere external, which was not searchable.
I recall someone had a similar issue, but then he never responded. The only possible solution to this is to either add exceptions – if possible in a form of regular expressions, so that some string contents including the JSON data are left as is without translation. I don’t know if weglot has a feature like that, I am not familiar with it.