It looks like you've got a URL parsing error or something similar when applying the sort key to your ENEQuery.
Do you have a scenario that triggers this or is it something you found when digging through your logs?
Also, what version is your MDEX/API? I tried to repro your error by giving a bad sort order to the reference app (and directly to the engine) but was unsuccessful at getting your exact error. Sometimes the error handling gets "tightened up" from release to release so I'm wondering if that was the reason.
Sort orders should be listed in the dgraph reqlog as <sortkey>|<order>, and the order can be 0 or 1. It looks like you've got a configuration error somewhere. If it definitely only happens after a partial update, then check:
1) <dgidx/> <args/> for --sort, is this set correctly? (Not sure if partial updates uses this, it might do)
2) Any cache warming script that runs as part of the PartialUpdate script
3) Check your dgraph.reqlog at the time the error is occurring, the actual request will be shown there and might provide more information as to the cause
4) Do a grep on the Endeca application for "|1" and see if this is set somewhere