According to http://build.fhir.org/search.html#table the data type 'string' should map to search parameters of type 'string' or 'token'.

However, the CarePlan resource has a search parameter of type 'date' named activity-date and this has an expression like "CarePlan.activity.detail.scheduled" which selects an element that can be a string (or a Timing or Period).

I suggest updating the search parameter expression to only select this element when its of type decimal (e.g. "CarePlan.activity.detail.scheduled as Period | CarePlan.activity.detail.scheduled as Timing")

Note: https://gforge.hl7.org/gf/project/fhir/tracker/?action=TrackerItemEdit&tracker_item_id=25173 identifies a different search parameter with the same issue (mapping string element to a date search parameter). if some global guidance is given in the search section, then maybe the expressions can stay as-is.